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8907 



Bureau of Mines Information Circular/1982 




Postdisaster Survival and Rescue 
Research 

Proceedings: Bureau of Mines Technology 
Transfer Seminar, Pittsburgh, Pa., 
November 16, 1982 

Compiled by Staff, Bureau of Mines 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8907 



Postdisaster Survival and Rescue 
Research 

Proceedings: Bureau of Mines Technology 
Transfer Seminar, Pittsburgh, Pa., 
November 16, 1982 

Compiled by Staff, Bureau of Mines 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Horton. Director 






This publication has been cataloged as follows: 



Bureau of Mines Technology Transfer Seminars (1982: 
Pittsburgh, Pa.) 

Postdisaster survival and rescue research. 

(Bureau of Mines information circular ; 8907) 

Includes bibliographical references. 

Supt. of Docs, no.: I 19.4/2:8907. 

1. Mine accidents— Congresses. 2. Mine rescue work— Con- 
gresses. I. United States. Bureau of Mines. II. Title. III. Series: 
Information circular (United States. Bureau of Mines) ; 8907. 

-T^29^e4 [TN311] 622s [622'.8] 82-600311 



PREFACE 

This Information Circular summarizes recent Bureau of Mines results 
covering postdisaster research. The papers are only a sample of the 
Bureau's total effort to improve mine health and safety through its 
Health and Safety Technology program, but they represent the major 
research effort in the area of postdisaster research. Those desiring 
more information on the Bureau's Mining Research program in general, or 
information on specific research, should feel free to contact the Bureau 
of Mines, Mining Research Directorate, 2401 E Street, NW, Washington, 
D.C. 20241, or the appropriate author listed in the following 
proceedings. 



CONTENTS 



111 



Page 



Preface i 

Abstract 1 

Introduction, by Sidney 0. Newman 2 

An Overview of Oxygen Self-Rescuer Technology, by John G. Kovac 3 

Laboratory Environmental Testing of Chemical Oxygen Self-Rescuers for Rugged- 

ness and Reliability, by Nicholas Kyriazi 18 

Chemical Oxygen Self -Contained Self-Rescuer Escape Study, by John G. Kovac, 

D. Randolph Berry, Diane M. Doyle, Elizor Kamon, and Donald W. Mitchell 32 

Medium Frequency Radio Communication System for Mine Rescue, by Harry Dobroski, 

Jr. , and Larry G. Stolarczyk 39 

Finding and Communicating With Trapped Miners, by S. Shope, J. Durkin, and 

R. Greenfield 49 

Bureau of Mines Borehole Probes Program, by James R. Means , Jr 79 

Mine Personnel Locator and In-Mine Activity Controller, by James R. McVey...... 84 



POSTDISASTER SURVIVAL AND RESCUE RESEARCH 

Proceedings: Bureau of Mines Technology Transfer Seminar, 
Pittsburgh, Pa., November 16, 1982 

Compiled by Staff, Bureau of Mines 



ABSTRACT 

These proceedings consist of papers presented at a Bureau of Mines 
Technology Transfer Seminar on postdisaster survival and rescue re- 
search. Several seminars are held each year to bring the latest results 
of Bureau research to the attention of the mining industry as quickly as 
possible. 



INTRODUCTION 
By Sidney 0. Newman'' 



The postdisaster research program is 
directed toward research and development 
of technology and equipment that in- 
creases the chances of a miner surviving 
or being rescued after an underground 
mine disaster. A. disaster is an accident 
of major proportions, and it may result 
in the entrapment of miners whose normal 
egress from the mine is cut off. This 
often necessitates a rescue operation and 
a means of keeping the trapped miners 
alive while they await rescue. The Bu- 
reau is currently pursuing research to 
develop the technology that will enhance 
the ability of miners to survive such an 
occurrence. The research is divided into 
two basic problem areas, survival and 
rescue. Both areas are concerned with 
the inability of miners and rescue teams 
to cope with the postdisaster environ- 
ment, such as toxic gases, unstable roof 
conditions, water flooding, and lack of 
oxygen. In addition, research is also 
conducted to find ways of locating and 
quickly reaching trapped miners. Most of 
the research conducted by the Bureau has 
been directed toward the postdisaster 
problems associated with coal mines. 
However, most of the research results 



Staff engineer, Postdisaster Research, 



Division of Health and Safety Technology, 
Bureau of Mines, Washington, D.C. 



are also applicable to noncoal mines as 
well. 

The papers presented in these proceed- 
ings address some of the recent research 
conducted by the Bureau of Mines that has 
been directed toward the postdisaster 
problems outlined above. The topics 
covered range from an overview of the 
technology developed for oxygen self- 
rescuers to training programs for mine 
rescue. Any questions or comments per- 
taining to this research are encouraged 
and appreciated. 

Open file report (OFR) references in 
the proceedings listed as available from 
NTIS may be obtained from the National 
Technical Information Service, Spring- 
field, Va. 22161, and are also available 
for reference at Bureau of Mines facili- 
ties in Denver, Colo., Twin Cities, 
Minn., Bruceton and Pittsburgh, Pa., and 
Spokane, Wash.; the Department of Energy 
facility in Morgantown, W. Va.; the Na- 
tional Mine Health and Safety Academy, 
Beckley, W. Va. , and the National Library 
of Natural Resources, U.S. Department of 
the Interior, Washington, D.C. 

Throughout the proceedings, mention of 
trade names is made to facilitate under- 
standing and does not imply endorsement 
by the Bureau of Mines. 



AN OVERVIEW OF OXYGEN SELF-RESCUER TECHNOLOGY 
By John G. Kovac^ 



ABSTRACT 



Federal regulations require that every 
person who goes into an underground coal 
mine in the United States be supplied 
with a self-contained self-rescuer 
(SCSR), a device capable of providing at 
least 60 min of oxygen regardless of 
ambient atmosphere. The development of 



oxygen self-rescuer technology suitable 
for in-mine use was a complicated engi- 
neering research and project management 
problem. The purpose of this paper is 
to trace the role that the Bureau of 
Mines played in the development of this 
technology from 1969 to the present. 



INTRODUCTION 



When a mine disaster occurs, the basic 
survival technique for a miner is to 
escape from the mine. Following a mine 
fire or explosion, the atmosphere inside 
a mine sometimes becomes oxygen deficient 
or filled with toxic gases. Under these 
circumstances, escape is nearly impos- 
sible unless a miner is equipped with a 
self-rescue device that supplies oxygen 
without the need of breathing mine air. 

Federal regulations (30 CFR 75.1714) 
require that every person who goes into 
an underground coal mine in the United 
States must be supplied with an SCSR. An 
SCSR is an emergency breathing apparatus 
designed for the purpose of mine escape. 
It must be capable of providing at least 
60 min of oxygen, regardless of the ambi- 
ent atmosphere. Only SCSR's approved by 
the National Institute of Occupational 
Safety and Health (NIOSH) and the Mine 
Safety and Health Administration (MSHA) 
can meet the provisions of the 
regulations. 

These regulations became effective on 
June 21, 1981, about 2.5 yr after they 



were promulgated by MSHA. Their success- 
ful implementation depended crucially on 
two factors: (1) the commercial avail- 
ability of approved SCSR's and (2) ac- 
ceptance by mine operators and mine work- 
ers that SCSR's were rugged enough to 
survive deployment underground and func- 
tion reliably in the event of a mine 
disaster. 

The purpose of this paper is to examine 
the role the Bureau of Mines played in 
the development of SCSR technology. The 
development process, translating a con- 
ceptual design for an oxygen self-rescuer 
into a workable SCSR technology, was a 
complicated engineering research and 
project management problem. This paper 
will describe how the Bureau of Mines 
successfully developed prototype SCSR's 
approved for in-mine use. These proto- 
types, because they derived from proven, 
available technology, demonstrated the 
feasibility of the SCSR concept, encour- 
aging manufacturers to adapt and mod- 
ify this technology for commercial 
production. 



SCSR DESIGN CONCEPTS 



In 1969 the National Academy of Engi- 
neering (NAE) established a Committee on 

- - , 

'Supervisory mechanical engineer, 

Pittsburgh Research Center, Bureau of 

Mines, Pittsburgh, Pa. 



Mine Rescue and Survival Techniques at 
the request of the Bureau of Mines. The 
purpose of this committee was to conduct 
a study program to assess technologies 
that could improve significantly a 



miner's chances for survival following a 
mine fire or explosion. ^ 

Based on a survey of underground coal 
mine disasters from 1950 to 1969, the NAE 
committee believed that a miner's chances 
for survival following a mine disaster 
could be significantly improved if he or 
she were equipped with a new type of 
emergency breathing apparatus, one that 
supplied oxygen without the need for 
breathing ambient mine air. Such a de- 
vice, they argued, was well within the 
reach of existing technology. Thus, the 
NAE committee recommended that the Bureau 
of Mines develop SCSR technology and 
described how an SCSR should optimally 
perform for mine escape purposes. 

The SCSR design requirements that 
emerged from the NAE committee's recom- 
mendations defined how an SCSR should 
optimally perform for mine escape pur- 
poses. There were nine design require- 
ments, as follows: 

1. Provide respirable atmosphere, re- 
gardless of environment. 

2. Permit intermittent voice communi- 
cations between miners. 

3. Supply 1 hr of oxygen at a work 
rate defined by 30 CFR 11, NIOSH Man 
Test 4. 

4. Should be lightweight and compact. 

5. Should be reliable and simple to 
operate, providing immediate safe oxygen 
levels at startup. 

6. Should be acceptable to miners. 



^Committee on Mine Rescue and Survival 
Techniques, National Academy of Engi- 
neering. Mine Rescue and Survival. 
Final Report (Contract SO190616). Bu- 
Mines OFR 4-70, March 1970, 81 pp.; NTIS 
PB 191 691. 



7. Should be rugged. 

8. Materials and design should be 
within present state of the art. 

9. Target costs should be less than 
$50 per unit (in 1969 dollars) for 50,000 
to 70,000 units produced within a 2-yr 
period. 

Besides defining how an SCSR had to 
optimally perform for mine escape pur- 
poses, the NA.E committee also described 
two conceptual designs for SCSR's, which 
they felt could potentially meet all nine 
design requirements. 

There are two ways to obtain oxygen in 
SCSR's: by storing oxygen physically as 
a compressed gas or a cryogenic liquid, 
or by generating oxygen chemically. From 
the start, liquid oxygen SCSR's were 
ruled out because such a storage system 
represented a high development risk. 
Chemical oxygen sources were favored over 
compressed oxygen because available bot- 
tled oxygen technology could not meet the 
compact size and weight requirements. 
Another reason for favoring chemical 
oxygen sources was that even if a com- 
pressed oxygen SCSR could be built, it 
was believed that a chemical oxygen SCSR 
would have fewer moving parts and, as a 
result, have lower maintenance require- 
ments and higher reliability. 

Both NAE designs used chemical oxygen 
sources, and, in order to meet the re- 
quirements of a 1-hr oxygen supply and 
compact size, both used a closed-loop 
breathing circuit. In a closed-loop 
breathing apparatus, all inhaled and ex- 
haled air is kept within the breathing 
circuit, conserving the available oxygen 
for reuse while requiring that the carbon 
dioxide produced by the body be absorbed. 
The NAE committee suggested that two dif- 
ferent oxygen sources be considered, 
a sodium chlorate candle plus a car- 
bon dioxide scrubber, and potassium 
superoxide. 



SCSR'S DEVELOPED BY THE BUREAU OF MINES 



The Bureau of Mines successfully de- 
veloped five different SCSR's through 
research contracts with private 
industries. 

In the order in which they were devel- 
oped, the five SCSR's are 

1. Westinghouse Electric Corp., Per- 
sonal Breathing Apparatus (PBA), 1971. 

2. Mine Safety Appliance Co., 10-min 
SCSR, 1973. 



3. Lockheed Missiles and Space Corp., 
PBA, 1974. 

4. MSA 10/60, 1978. 

5. U.S. Divers Self-Contained Emer- 
gency Breathing Apparatus 60 (SCEBA 60), 
1981. 

All five devices are shown in figure 1. 
The size, weight, and operating charac- 
teristics of each device are listed in 
table 1. 



TABLE 1. - Specifications of Bureau-developed SCSR's 



SCSR 


Duration, 


Carrying weight, 


Volume , 


Oxygen source 




mxn 


lb 


cu ia 




Westinghouse PBA 


60 


8.7 


525 


Chlorate candle. 


Lockheed PBA. . . . 


60 


4.5 


200 


Potassium superoxide. 


MSA 10-min SCSR. 


10 


2.4 


100 


Do. 


MSA 10/60'' 


10 


4.2 


170 


Do. 




60 


8.5 


NAp 




SCEBA 60 


60 


6.2 


354 


Compressed oxygen. 



NAp Not applicable. ^Deployed together. 




FIGURE 1. . Bureau-developed SCSR's. Left to right: SCEBA 60, MSAlO/60, MSA lO-min SCSR, 
Lockheed PBA, Westinghouse PBA. 






FIGURE 2o - Westinghouse PBA as worn by a miner for escape. 



Westinghouse Electric Corp., PBA 

The Bureau of Mines funded Westinghouse 
Electric Corp. to develop a 1-hr, sodium 
chlorate candle SCSR.3 

The Westinghouse PBA was a straightfor- 
ward feasibility demonstration of the 
SCSR concept using available technology 
as a starting point. It met most of the 
optimum design requirements. While the 
Westinghouse PBA was made as small as 
possible within the state of the art for 
sodium chlorate candles. Its overall size 
and weight made It impractical for miners 
to carry this SCSR constantly. Figure 2 
shows the Westinghouse PBA In use on a 
miner. 

Since the Westinghouse PBA was a closed 
circuit breathing apparatus, it used pure 
oxygen as the breathing medium, generat- 
ing oxygen by burning a sodium chlorate 
candle. The sodium chlorate candle was 
ignited automatically when the SCSR was 
pulled from its carrying container. 

The flow path through the Westinghouse 
PBA is shown in figure 3. The wearer 
breaths oxygen to and from breathing 
bags. Exhaled gas passes through a 
canister containing a carbon dioxide 

absorbent lithium hydroxide. Fresh 

oxygen from the sodium chlorate candle is 
constantly added to the breathing cir- 
cuit. The amount of oxygen added will 
support a person working at the hardest 
work possible. Therefore, more oxygea is 
generated than is normally used by a per- 
son not working at maximum effort. The 
oxygen not used is vented through a one- 
way relief valve in the right breathing 
bag. This relief valve vents at a very 
low pressure buildup, and therefore, nor- 
mally vents most of the time the appara- 
tus is in use. 

Separate tubes provide for inhalation 
and exhalation. Breathing check valves 
in the mouthpiece maintain one-way flow 

■^Westinghouse Electric Corporation. 
Coal tllne Rescue and Survival. Volume 1. 
Survival Subsystem (Contract HO101262). 
BuMines OFR9(1)-72, September 1971, 
113 pp.; NTIS PB 208 266. 



Inhalation 
check valve 




Sodium chlorate candle 
oxygen (O2) source 



"Fresh O2 



FIGURE 3.- Flow path through the Westinghouse PBA. 

of the oxygen, preventing the wearer re- 
breathing gas from which carbon dioxide 
has not been removed. Rebreathing does 
not occur until the exhaled gas has been 
passed through the lithium hydroxide. 

A plastic hood with a rubber neck seal 
and antlfogging goggles is sealed around 
the mouthpiece to provide eye protection 
to the miner, to permit the miner to re- 
move the mouthpiece without admitting 
contaminated air, and to ensure universal 
fit of the apparatus. A nose clip on the 
eyepiece holds the nose shut to prevent 
breathing from the hood. 

MSA lO-Mln SCSR 

Because of the size of the Westinghouse 
PBA, the Bureau of Mines funded two sep- 
arate contracts for the development of a 
10-min, belt wearable SCSR and a 1-hr 
SCSR using potassium superoxide as the 
oxygen source. The development of the 
MSA 10-min SCSR will be discussed first.'* 

'^Buban, E. E., and R. E. Gray. Short 
Duration Self-Rescue Breathing Apparatus 
(Contract HO220071, Mine Safety Appliance 
Co.). BuMines OFR 6-75, Apr. 1, 1974, 
120 pp.; NTIS PB 240 471. 




FIGURE 4. - MSA 10-min SCSR as worn by a miner for escape. 



Figure 4 shows the MSA 10-min SCSR de- 
ployed for use. In order to meet belt- 
wearability requirement, this SCSR was 
designed to use a pendulum breathing 
circuit. 

Figure 5 is a drawing of the flow path 
through the MSA 10-min SCSR. The exhaled 
air goes through the potassium superoxide 
bed where oxygen is produced and carbon 
dioxide is absorbed; then the exhaled air 
goes into the breathing bag. On inhala- 
tion, the gas from the bag returns by way 
of the same route. The split potassium 
superoxide bed design was chosen to make 
maximum use of the potassium superoxide 
while keeping overall breathing resist- 
ance low. The pressure relief valve on 
the breathing bag is necessary because 
the potassium superoxide produces slight- 
ly more oxygen than is needed. For safe- 
ty, the potassium superoxide bed is 
designed to absorb all carbon dioxide, 
and thus, overproduces oxygen. The re- 
lief valve is a one-way valve so that no 
toxic gases can enter the breathing bag. 



Because potassium superoxide does not 
provide oxygen instantly, a supply of ox- 
ygen is provided by a sodium chlorate 
candle for the first 45 sec. This helps 
inflate the breathing bag and provides 
the oxygen the wearer would need for the 
first minute or so. By that time enough 
breath moisture will have reacted with 
the potassium superoxide so as to provide 
the oxygen needed. 

Inside the plastic carrying case, the 
MSA 10-min SCSR is packed in a double- 
sealed vapor bag. After the sealed bag 
is opened, the SCSR is donned by putting 
the bag strap over the neck. The sodium 
chlorate candle is started automatically 
by pulling a firing pin that is attached 
to the goggles used to protect the eyes 
from smoke. The entire donning operation 
can be accomplished in less than 30 sec. 

The results of personnel tests of the 
MSA 10-min SCSR showed that the device 
would last at least 10 min or longer 
depending on work rate. 



Relief valve 



Breathing bag 



Filtered KO2 bed 
Heat exchanger 





Chlorate candle 



Mouthbit 



KO2 canister 



Breathing bag 

FIGURE 5. - Flow path through the MSA lO-min SCSR. 



10 




FIGURE 6. - Lockheed PBA as worn by a miner for escape. 



LOCKHEED PBA 



11 



In a parallel effort, the Bureau of 
Mines funded a contract with Lockheed 
Missiles and Space Corp. to develop 1-hr 
potassium superoxide SCSR.^ 

The Lockheed PBA is shown in figure 6 
and a flow path diagram is shown in fig- 
ure 7. In this SCSR the wearer exhales 
through the exhalation breathing tube 
down through the potassium superoxide bed 
and into the breathing bag. On inhala- 
tion, oxygen enriched air scrubbed of 
carbon dioxide bypasses the potassium 
superoxide bed by way of the return duct, 
then enters the inhalation breathing tube 
and passes into the mouthpiece; check 
valves at the mouthpiece assembly control 
the inhaled and exhaled direction of 
flow. Again, a relief valve on the 
breathing bag is needed to vent excess 
oxygen. 

In order to keep inhaled air tempera- 
ture within NIOSH requirements (<115° F), 
the heat generated by the chemical reac- 
tion of potassium superoxide is removed 
by gas flow routed through the breathing 
bag by internal baffles. The large sur- 
face area of the breathing bag exchanges 
heat with the ambient mine air. 



Mouthpiece 
with checic 
valves 




KO2 bed with 
screens 



KO2 
catcher 



Baffles 



Relief valve 

~^— Breathing 
bag 



FIGURE 7. - Flow path through the Lockheed PBA. 

to determine if the outer case has leaked 
moisture. 

When the latch mechanism is opened, the 
bottom cover falls away, and the breath- 
ing bag deploys from the bottom. The top 
cover is placed between the wearer and 
the unit in order to keep the hot outer 
cover from touching the wearer. This 
also allows mine air to surround the 
apparatus and keep it cool. 



A small sodium chlorate candle provides 
instant oxygen, similar to the MSA 10-min 
SCSR. This candle supplies 2 liters of 
oxygen in the first 15 sec and 8 to 
10 liters during the 90 sec that follow. 
Pulling the mouthpiece towards the wearer 
automatically fires the sodium chlorate 
candle. 

The top and bottom covers on the Lock- 
heed PBA are held in place with "0" ring 
seals and a band strap. The outer case 
and cover provide moisture and shock pro- 
tection for the SCSR. The moisture indi- 
cator in the upper cover allows a wearer 



^Shengli, Y., and E. N. Perry. One- 
Hour Self-Rescue Breathing Apparatus 
(Contract HO220040, Lockheed Missiles and 
Space Corp.). BuMines OFR 8-75, October 
1974, 123 pp.; NTIS PB 240 420. 



During a demonstration wear test of the 
Lockheed PBA, one unit had the breathing 
bag catch fire. This was caused by 
potassixim superoxide escaping from the 
chemical bed and bypassing the potassium 
superoxide catcher, falling into the 
breathing bag. The combination of 
thermally hot potassium superoxide with a 
silicon-fiberglass breathing bag in the 
presence of about an 80-pct oxygen atmos- 
phere was enough to start the fire. 

Despite this serious problem and other 
design flaws, such as the need for 
protective goggles to be packaged in- 
side the unit, the Lockheed PBA became 
the prototype for commercially avail- 
able, NIOSH-MSHA approved SCSR's. Both 
MSA and Draeger refined and successfully 
commercialized Bureau developed SCSR 
technology, producing the MSA 60-min SSR 
and the OXY SR 60B, respectively. 



12 




FIGURE 8. - MSA 10/60 SCSR as worn by a miner for escape. 



13 



MSA 10/60 

The necessity of discarding the MSA 
lO-min SCSR in order to don a 1-hr SCSR 
in a contaminated environment was consid- 
ered by the Bureau of Mines to be a de- 
sign drawback. To overcome this limita- 
tion, it was decided that an optimum 
system would combine a 10-min, belt- 
wearable SCSR with a stored, larger, 1-hr 
duration oxygen supply that could be 
plugged into the 10-min unit without 
removing the mouthpiece. This 10/60 de- 
sign concept was developed into a work- 
able technology by Mine Safety Appliance 
Co., under contract to the Bureau of 
Mines. ^ 

Figure 8 shows the MSA 10/60 SCSR as 
used by a miner for escape purposes. The 
1-hr oxygen supply is plugged into the 
10-min unit. 

For the 10-min unit, a breathing cir- 
cuit was developed in which the air is 
passed through the potassium superoxide 
bed twice per respiration. The 1-hr oxy- 
gen supply differs from the 10-min unit 
in that air is drawn only once through 
the chemical bed per respiration. In 
other words, after connecting both com- 
ponents together, the breathing circuit 
is switched from a pendulum system 
to a simple closed loop system. This 
was necessary to keep the inhaled 
air temperature within NIOSH approval 
requirements. 

A 6-cm finned aluminum cylinder located 
in the breathing tube acts as a heat ex- 
changer in the 10-min device. Location 
of the heat exchanger in the breathing 
tube also prevents the breathing tube 

^ine Safaty Appliances Co. Combined 
Short and Long Duration Rescue Breathing 
Apparatus (Contract HO252079) . July 
1976; available for consultation at Bu- 
reau of Mines Pittsburgh Research Center, 
Pittsburgh, Pa. 



from closing. The breathing bag and the 
manifold in the 1-hr oxygen supply are 
used as the primary heat exchanger when 
both components of the MSA 10/60 
are assembled together. These heat 
exchange mechanisms lower inhalation 
air temperature to less than 115° F, 
meeting NIOSH certification requirements. 
Both potassium superoxide canisters have 
a felt cover to protect the user from 
burns . 

Operation of the MSA 10/60 SCSR in- 
volves connecting the 10-min and 1-hr 
components together. Figure 9 shows the 
flow path through the assembled system. 




— ^ Inhalation 
•• Exhalation 



FIGURE 9. - Flow path through the MSAlO/60 SCSR. 



14 



The manifold system Is designed to 
limit the Inhaled concentration to less 
than 50 ppm of carbon monoxide If the 
coupling Is performed In a mine atmos- 
phere contaminated with 2.5 pet carbon 
monoxide. The potassium superoxide can- 
isters have hermetic seals over their 
coupling ports. When the 1-hr oxygen 
supply Is pressed against the lO-mln 
unit, a metal-to-metal seal Is made 
between the ducts prior to opening of 
either of the Individual hermetic seals. 
Initial rotation of the handle on the 
latch assembly secures the metal-to-metal 
seal between the two components. Further 
rotation punctures the hermetic seals and 
activates the shutoff valve for the 
10-mln unit. 

The results of breathing machine and 
personnel tests Indicated that the MSA 
10/60 SCSR will last 60 to 270 mln, 
depending on work rate. 

SCEBA 60 



(JO100092) to U.S. Divers to develop a 
1-hr compressed oxygen SCSR suitable for 
mine escape use. 

The SCEBA 60 as used by a miner for 
escape Is shown In figure 10. The 
apparatus Includes a mouthpiece, nose 
clip, breathing hose and bag, a 
lightweight, single-use high-pressure ox- 
ygen vessel (115 liters at 3,000 psi) , 
lithium hydroxide absorbent canister, and 
a pressure reducing on-off valve. 

The SCEBA 60 uses a pendulum flow cir- 
cuit shown schematically In figure 11, 
which provides a push-pull action through 
the carbon dioxide absorbent bed. Just 
as In the case of chemical oxygen SCSR's, 
to minimize size, the SCEBA 60 Is a 
closed circuit breathing apparatus with 
an external breathing bag being used for 
gas storage. A compressed oxygen ves- 
sel and associated regulating valves 
control the addition of oxygen to the 
system. 



Given the experience and technology of 
the late 1960's and early 1970's, the 
development of compressed oxygen SCSR's 
held little promise of success. This Is 
the chief reason why the Bureau of Mines 
Invested research and development re- 
sources In developing chemical oxygen 
SCSR technology. 

In the late 1970' s, however. It became 
clear that the state of the art In com- 
pressed gas breathing apparatus had Im- 
proved considerably over the past 10 yrs. 
This technology had advanced to the point 
where a compressed oxygen SCSR could be 
offered as a viable alternative to potas- 
sium superoxide SCSR's. Therefore, the 
Bureau of Mines awarded a contract 



In addition to these major components, 
the SCEBA 60 Includes an easily reli- 
able pressure gage, a volume sensing 
relief valve, and a sealed carry case 
with quick donning waist and neck 
straps. 

At present, the SCEBA 60 Is undergoing 
NIOSH certification trials. 

Based on Bureau-developed technology, 
MSA and Draeger refined the design of the 
Lockheed PBA into commercial products; 
the Draeger OXY SR 60B and the MSA 60-mln 
SSR. The SCEBA 60 SCSR will probably be 
commercialized once it receives NIOSH 
approval for in-mine use. 



15 



, ^ 

r.^ 




.'# «SS 




FIGURE 10. - SCEBA 60 as worn by a miner for escape. 



16 



Mouthpiece- 
breathing tube 



O2 storage bag 



Constant flow demand 
regulator inside bag 



CO2 volume control- 
relief valve 



Outer case 




O2 storage bag 



O2 pressure gage 
and valve 



O2 supply 



CO2 absorbant 
canister 



KEY 

^^ CO2 fully removed 
100 pet O2 

1=^ CO2 partially removed 

c=> Expired CO2 laden gas 



FIGURE 1L = Flow path through the SCEBA 60. 



STATE OF THE ART IN SCSR TECHNOLOGY 



Five models of NOISH-MSHA approved 1-hr 
SCSR's are commercially available — CSE 
AU9-A1, Draeger OXY SR 60B, MSA 60-min 
SSR, OCENCO EBA 6.5, and PASS 700E. All 



five devices are shown in figure 12. The 
size, weight, and operating character- 
istics of each SCSR are listed in 
table 2. 



TABLE 2. - Specifications for commercially available, approved 1-hr SCSR's 



SCSR 


Weight, lb 


Volume, 
cu in 


Oxygen source 




Carrying 


Deployed 




CSE AU9-A1 


11.0 
8.4 
9.1 
7.7 

19.0 


9.5 

7.4 

6.7 

6.8 

14.5 


354 
366 
360 
452 
757 


Compressed oxygen. 
Potassium superoxide. 
Do. 


Draeger OXY SR 60B 

MSA 60-min SSR 


OCENCO EBA 6.5 

PASS 700E 


Compressed oxygen. 
Do. 



17 




FIGURE 12. = Commercially available approved SCSR's. Left to right: (top) Draeger OXY=SR 60B, 
PASS 700, SCEBA 60, (bottom) CSE AU9=A1, MSA 60=min SSR, OCENCO EBA 6,5o 



In order to meet the 1-hr duration 
requirement, all of the SCSR's are closed 
circuit breathing apparatus. Both the 
Draeger OXY SR 60B and the MSA 60-min SSR 
use potassium superoxide to generate ox- 
ygen and remove carbon dioxide. The CSE 
AU9-1, OCENCO EBA 6.5, and PASS 700E 
store oxygen as a compressed gas and use 
lithium hydroxide to absorb carbon 
dioxide. With the exception of the PASS 
700E, all of the SCSR's can be worn 



by the miner as personal protective 
equipment; the PASS 700E must be carried 
or stored. 

In 1982 the cost of an approved SCSR 
is about $500. Taking inflation into 
account, the target cost of $50 per SCSR 
projected by the NAE in 1969 becomes 
about $150 per SCSR in 1982. So the 
projected actual SCSR costs differ by 
about a factor of three. 



CONCLUSIONS 



The Bureau of Mines successfully pur- 
sued the development of SCSR technology, 
cooperating with private industry to pro- 
duce three prototype SCSR's approved for 
in-mine use. 



Manufacturers refined and adapted 
Bureau-developed SCSR technology for com- 
mercial production. 



LABORATORY ENVIRONMENTAL TESTING OF CHEMICAL OXYGEN SELF-RESCUERS 
FOR RUGGEDNESS AND RELIABILITY 



By Nicholas Kyriazi^ 



ABSTRACT 



The Bureau of Mines subjected two manu- 
facturers' chemical oxygen (KO2 ) self- 
rescue breathing apparatus to a series of 
laboratory environmental treatments 
designed to simulate various conditions 
in underground coal mines. The environ- 
mental treatments consisted of extremes 
of temperature, shock, and vibration. 
The tests were designed to be used as 
predictors of the ability of the self- 
rescuers to survive those environmental 
insults with no degradation in their pro- 
tection to the wearer. 



Based on the severity of the treat- 
ments, simulating conditions more severe 
than offered by the mining environment, 
the two apparatus tested should be able 
to withstand the abuse offered by the 
mining environment and still function as 
intended in an emergency. Correlation of 
these tests with results of long-term 
field evaluations is needed to provide 
confidence in the laboratory tests as 
predictors. 



INTRODUCTION 



On June 2i, 1981, coal mine operators 
were required to make available to each 
underground coal miner in the United 
States a self-contained oxygen self- 
rescuer (OSR). The regulations, 30 CFR 
75.1714, require that each person in an 
underground coal mine wear, carry, or 
have immediate access to a self-rescuer 
that provides an oxygen source. The OSR 
will replace filter self-rescuers (FSR) 
as primary escape equipment. FSR's pro- 
tect only against low levels of CO. 

In December 1980 the Bureau began labo- 
ratory environmental testing of the two 
chemical oxygea OSR's that had been 
approved by the National Institute for 
Occupational Safety and Health (NIOSH) 
and the Mine Safety and Health Admin- 
istration (MSHA), the Draeger OXY-SR 60b2 
and Mine Safety Appliances (MSA) 60- 
minute self-contained self-rescuers 
(SCSR); no other self-rescuers had NIOSH- 
MSHA approval at that time. The purpose 
of this series of environmental tests was 



T 



'Biomedical engineer, Pittsburgh Re- 
search Center, Bureau of Mines, Pitts- 
burgh, Pa. 

^Reference to specific products does 
not Lmply endorsement by the Bureau of 
Mines . 



to attempt to project the effects of the 
underground mining environment on OSR's. 
Such studies are not always done on new 
equipment before large-scale deployment, 
but because OSR's are used for life sup- 
port, it is critical that they be main- 
tained in operable condition. These 
laboratory tests were planned to give 
further knowledge and assurances about 
the readiness of OSR's to operate when 
needed. 

There is no implication that either 
NIOSH, MSHA, or the manufacturers have 
conducted less than thorough testing of 
these devices. There is a high level of 
confidence that they are dependable 
devices. However, the need to study 
environmental effects arises owing to the 
gradual deterioration that all equipment 
and materials are expected to experience. 
Environmental testing can help estimate 
equipment lifetime. For OSR's, the 
questions of concern are related to ex- 
pected lifetime in various modes of sec- 
tion (container) storage, placement on 
mining equipment, and wear or carry. 

The assured answers to these questions 
can come only from experience, and the 
Bureau intends to perform a long-term 
field evaluation after actual deployment 



19 



of the OSR's. In lieu of this experi- 
ence, the laboratory tests offer the fol- 
lowing benefits: (1) If the test is 
severe eaough, one can directly observe 
the failure mode for a particular 
environmental assault on the equipment; 
and (2) the laboratory test results can 
be used as indicators of areas where 



attention should be focused during the 
field evaluations. 

The test results should not be applied 
to other OSR's or breathing apparatus, 
since they are specific to the two chemi- 
cal O2 devices evaluated. 



UNIT OF MEASURE ABBREVIATIONS USED IN THIS PAPER 



bpm breaths per minute 

cu in cubic inches 

° C degrees Celsius 

ft/sec feet per second 

hr hours 



kg 


kilograms 


min 


minutes 


lb 


pounds 


mm 


millimeters 


Ipb 


liters per breath 


mph 


miles per hour 


1pm 


liters per minute 


sec 


seconds 


m 


meters 


yr 


years 



DESCRIPTION OF SELF-RESCUERS 



Basically, a closed circuit, self- 
contained breathing apparatus of any type 
is composed of a mouthpiece or facepiece 
and breathing hose, an oxygen source, a 
carbon dioxide absorbent, and a breathing 
bag. In the case of chemical oxygen 
apparatus, KO2 is both the oxygen source 
and the carbon dioxide absorbent, satis- 
fying human physiological needs with a 
little excess oxygen. This excess oxygen 
has the effect of continually purging the 
system of nitrogen, which may exist in 
the breathing loop. 

As can be seen from figures 1 and 
2, the MSA 60-min SSR and Draeger 
OXY-SR 60B apparatus have different 
flow paths and arrangements of com- 
ponents. Their functions, however, are 
the same — to provide a portable life- 
supporting atmosphere. The oxygen self- 
rescuers differ from the filter self- 
rescuers (fig. 3) in that they are bigger 
and heavier. Inhalation temperatures for 
SCSR's may be high towards end of 
apparatus life, but this is unrelated to 
ambient conditions, whereas the FSR 
may heat up to unbearable tempera- 
tures and actually burn the lips 
of the user if ambient CO concentrations 
are high. Duration is determined in 
FSR's by ambient CO concentrations, hu- 
midity, and physiological demand; in 
SCSR's the physiological demand of the 
user alone determines duration, and an 



SCSR may last as long as 5 hr if the user 
is sedentary (NIOSH man-test 5). 




/ Plug, mouthpiece 

2 Valve box 

J Upper valve chamber 

4 Hear exchanger 

5 Sieve 

6 Inhalalion valve disks 



KEY 

/■ Exhalation valve 

8 Hose clamp 

9 Chemical cartridge KOg 
/O Starter (chlorate candle) 
// Split pin 

/2 Ripcord 



I ^"xj Oxygen (Og) 



/J Breathing bag 

/4 Relief valve 

/5 Central hose 

/6 Lower valve chamber 

// Nose clip 

/8 Corrugated hose 

/9 Mouthpiece 



Carbon dioxide (CO2) 



FIGURE 1. - Draeger oxygen self-rescuer. 



20 



BreoThing tube 




Head straps 



Noseclip 



KEY 

^ Inhalation flow 
Exhalation flow 



FIGURE 2. - MSA oxygen self-rescuer. 

EXPERIMENTAL DESIGN AND TEST METHODS 

Laboratory testing consisted of 
(1) determining baseline performance for 
untreated, new OSR's, (2) performing en- 
vironmental degradation treatments, and 
(3) measuring the effects of the treat- 
ments on OSR operational lifetime. The 
treatments performed were temperature ex- 
tremes (71° C, 48 hr; 100° C, 4 hr; and 
-45° C, 16 hr) and shock and vibration. 
Both human subject tests on a treadmill 
and machine tests using a breathing and 
metabolic simulator (BMS) were used to 
measure the effects of the environmental 
treatments on OSR performance. Human 
subject testing provided relevant human 
factors information, while the BMS tests 
provided a more reproducible method for 
quantifying the duration of respiratory 
protection. Duration of respiratory pro- 
tection is necessarily a function of the 
workloads performed during testing. The 




Hopcalite catalyst 

(converts toxic CO to 

nontoxic COg ) 



Drying agent 
removes moisture to 
prevent contamination 
of catalyst) 



Fine dust filter 
(removes fine particles) 



Coarse dust filter bag 
removes large particles) 



KEY 

C^:^ Inspired 
^M^^ Expired 



FIGURE 3, - Filter self-rescuer (FSR). 



BMS, unlike a human subject, can be 
programmed to precisely reproduce a given 
demand (work load) from test to test. 

An apparatus could fail in two ways: 
Measured parameters could exceed prede- 
fined limits, or the apparatus could 
cease to support life completely. In 
other words, even though an apparatus may 
be very hard to breathe through, for 
example, and may exceed the predefined 
limits, it could still be used in a life 
and death situation to escape from an 
irrespirable atmosphere. The difference 
between these two definitions was recog- 
nized and noted in some cases in this 
s tudy . 



21 



TREADMILL TESTING 



The human subject test used was the 
treadmill equivalent of NIOSH man-test 4 
for 60 min (table 1). The treadmill sim- 
ulation of the test was based on the pub- 
lished studies of Kamon,-^ Treadmill 
testing permitted continuous monitoring 
of CO2 and O2 measured in the breathing 
bag, and temperature and pressure mea- 
sured in the mouthbit. At the end of 
60 min, a constant speed chosen by each 
subject was maintained until apparatus 
failure. Three human subjects were used 
for the treadmill testing. Character- 
istics of untreated, new GSR's were mea- 
sured during human subject and machine 
tests to establish the normal range of 
performance. These tests were used as 
controls for comparison with the treated 
GSR's. Since each subject would put a 
different demand on the apparatus, owing 
to differing weights and end-run speeds, 

^Kamon, E., T. Bernard, and R. Stein. 
Steady State Respiratory Responses to 
Tasks Used in Federal Testing of Self- 
Contained Breathing Apparatus. Am. 
Ind. Hygiene Assoc. J., December 1975, 
pp. 886-896. 



these factors were normalized by compar- 
ing the duration of each subject's 
treated units only with his or her own 
control unit. Normalization consisted of 
dividing each duration for a treated unit 
by the duration of the control unit. 
This was done for each test subject so 
that each control apparatus would have a 
normalized duration value of 1.00 with 
the treated units having normalized dura- 
tion values varying around 1.00. Weights 
of the subjects and their end-run speeds 
are given in table 2. Duration was de- 
fined by the termination time. Factors 
determining termination were inhaled gas 
concentrations of CO2 greater than 1.5% 
or of G2 less than 21%, inadequate gas 
volume (bag bottoming on inhalation) , any 
subjective intolerable discomfort such as 
breathing resistance or high temperature 
of inhaled gas or of apparatus surface, 
or an excessively high heart rate 
(greater than 90% of maximum) . If a 
treated unit reached the duration of a 
control test, the test was usually 
stopped for the benefit of the test 
subject. 



TABLE 1. - NIOSH man-test 4 and treadmill equivalent 



Time , min ' 



NIOSH man-test 4^ 



Treadmill equivalent 



Sampling and reading 

Walk at 3 mph 

Climb vertical treadmill (1 ft/sec) 



Walk at 3 mph 

Pull 45-lb weight to 5 ft (60 times in 5 min) 

Walk at 3 mph 

Carry 50-lb weight over overcast (4 times in 8 min) 

Sampling and reading 

Walk at 3 mph 

Run at 6 mph 

Carry 50-lb weight over overcast (6 times in 9 min) 

Pull 45-lb weight to 5 ft (36 times in 3 min) 

Sampling and reading 

Walk at 3 mph 

Pull 45-lb weight to 5 ft (60 times in 5 min) 

Carry 45-lb weight and walk at 3 mph 

Sampling and reading 



Stand. 

Walk at 3 mph. 

Walk at 4.5 mph at 

15% grade. 
Walk at 3 mph. 
Walk at 4.2 mph. 
Walk at 3 mph. 
Walk at 2.7 mph. 
Stand. 

Walk at 3 mph. 
Run at 6 mph. 
Walk at 3.6 mph. 
Walk at 4.2 mph. 
Stand. 

Walk at 3 mph. 
Walk at 4.2 mph. 
Walk at 4.2 mph. 
Stand. 



Overall test takes 1 hr. ^30 CFR 11-H. 



22 



TABLE 2. - Human subject weights and 
end-run speeds 



Subject 


Weight, kg 


Constant end-run 
speed, mph 


A 


49 
68 
73 


3 


B 


6 


C 


6 



More than one test for each treatment 
and control per person were not run for 
several reasons. Additional testing on 
the BMS of the treated units was planned 
and, taken with the treadmill testing, 



was felt to be sufficient. The treadmill 
testing results cannot be considered 
definitive if taken alone, however. 
More control tests per person were not 
run since previous experience with lab- 
oratory testing of the OSR's showed our 
control tests to be typical. Also, 
the physiological demand for a human sub- 
ject varies with changes in running 
style, weight, fitness, and diet. This 
would preclude any reliance on human sub- 
ject testing for providing reproducible 
physiological demand. This was the pur- 
pose of the BMS testing. 



BREATHING METABOLIC SIMULATOR TESTING 



A prototype breathing metabolic simu- 
lator built by Reimers Consultants, Falls 
Church, Va. , was used in the machine- 
testing part of the study (fig. 4). The 
metabolic state used in the machine test- 
ing represented the average work rate 
that would be exhibited by a 50th per- 
centile miner (87 kg) performing man- 
test 4 for 60 min.^ The physiological 
parameters at standard temperature and 
pressure, dry, follow: 



Vo„ ~ Oxygen consumption - 1.35 1pm 

Vqo^ - Carbon dioxide production - 
1.30 1pm 



Ve - Ventilation - 31.89 1pm 

Vj - Tidal volume - 1.21 Ipb 

RF - Respiratory frequency - 26.5 bpm 

Termination factors were inhaled gas con- 
centrations with more than 1.5% CO2 , or 
less than 21% O2 , or inadequate gas vol- 
ume. For a treatment to be considered to 
have had no impact on an apparatus , the 
chlorate candle must function and there 
must be no significant degradation in the 
duration compared to the control tests. 
A discussion of the various environmental 
treatments and methods follows. 



SHOCK AND VIBRATION TREATMENT 



There is no specific NIOSH or MSHA re- 
quirement in the Code of Federal Regula- 
tions for shock or vibration testing of 
breathing apparatus. At present, how- 
ever, NIOSH requires that self -rescuers 
survive 40 hr of shock and vibration on a 
Rotap sieve shaker. The two chemical 
OSR's tested by the Bureau had success- 
fully passed this test during NIOSH 
and/or MSHA approval testing. 

The Rotap machine subjects the OSR to 
vibration from rotary motion and an im- 
pact from a hammer blow (2.5 impacts/ 
sec). The OSR is rigidly mounted to 

^Work cited in footnote 3. 



avoid excessive accelerations and moni- 
tored to maintain accelerations within 
15 g, peak to peak, for the entire test 
period. The test's origin is from expe- 
rience with FSR's and simulates the 
extent of damage suffered in worst case 
tests of harsh mining environments, in- 
cluding carrying and mounting on machines 
for 1 yr. The Rotap test itself, though, 
does not simulate vibration spectra and 
types likely to be seen on mining machin- 
ery. To resolve this problem, we devised 
a composite test based on the reported 
vibration levels experienced on portable 
equipment , on underground mining machines 
(long-wall, continuous) measured on the 
frame, and on underground and surface 



23 




FIGURE 4. - Breathing metabolic simulator. 



24 



haulage vehicles.^ A shaker table of the 
type used in military standard (MIL-STD) 
vibration tests was used in the vibration 
treatment with motion along the vertical 
(Z) axis only (fig. 5). The test condi- 
tions are as follows: 



Frequency , 
Hz 

5 - 92 

92 - 500 

500 - 2,000 



Acceleration, 
g(± peak) 

2.5 
3.5 
1.5 



There is no consensus on what constitutes 
an appropriate vibration treatment simu- 
lating the mining environment. MIL-STD 

Dayton T. Brown, Inc. Environmental 
Test Criteria for the Acceptability of 
Mine Instrumentation. USBM Contract 
J01 0040, June 1980; available for con- 
sultation at the Pittsburgh Research Cen- 
ter, Bureau of Mines, Pittsburgh, Pa. 



810B, which specifies a frequency range 
of 9 to 500 Hz at an acceleration of 4 g 
(± peak), has been recommended, but 
others recommend MIL-STD 810C,^ which 
specifies 1.5 g (± peak) from 5.5 to 
30 Hz, increasing to 4.2 g (± peak) at 
5 to 500 Hz , as being a more appropriate 
test. 

"Bolt, Beranek and Newman, Inc. Shock 
and Vibration Tests for Mining Machin- 
ery Instrumentation. BuMines Contract 
H0155113, Addendum to Report No. 40 33. 
January 1979; available for consultation 
at the Pittsburgh Research Center, Bureau 
of Mines, Pittsburgh, Pa. 

"^Berry, D. R. , and D. W. Mitchell. 
(Foster-Miller Associates) . Recommended 

Guidelines for Oxygen Self-Rescuers — Vol- 
ume 1 , Underground Coal Mining. BuMines 
Contract J0199118; available for consul- 
tation at the Pittsburgh Research Center, 
Bureau of Mines, Pittsburgh, Pa. 




FIGURE 5. - Vibration treatment equipment. 



25 



One variation on vibration tests which 
we made was to vibrate the self-rescuers 
loose rather than strapping them down as 
is usually done. We felt that, based on 

Q 

European experience, the self -rescuers 
would not be strapped tightly to 
machines, but, rather, simply placed in 
unpadded holders if not just thrown on 
the floor or other surface, unrestrained. 
We restricted their lateral motion 
(±1 cm) with pegs screwed into the 1.3-cm 
aluminum table. While at first inspec- 
tion it would seem that the bouncing 
of the apparatus at lower frequencies 
would make individual treatments vastly 
different in terms of vibration and shock 
insult, we believe that the cumulative 
effect of the undamped vibration treat- 
ment over an entire test is similar and 
reproducible. 

_ 
Work cited in footnote 7. 



The control accelerometer was screw- 
mounted as close to the OSR on the table 
as possible without any danger of contact 
with it. With another accelerometer 
glue-mounted on top of the self-rescuer 
so as to be sensitive to motion in the 
Z-axis, we did resonance searches at an 
input of 1 g (± peak). At most three 
resonances, but usually fewer or none, 
were noted. A resonance was defined as 
being consistent regardless of self- 
rescuer orientation and greater than 2 g 
(± peak). For each resonance, the self- 
rescuer was vibrated at the appropriate 
g level for 30 min. The remaining test 
time was spent sweeping the frequency 
range with a sweep time of 20 min for a 
total test time of 3 hr. This procedure 
was performed for each axis for a total 
vibration test duration of 9 hr 
(fig. 6). 






FIGURE 6o - Draeger OXY-SR 60B, showing the three orientations used in the vibration treatment. 



26 



The shock portion of the treatment was 
a drop of 1 m (belt-height) onto a con- 
crete floor. This was performed on each 



axis also. We consider this to be a 
worst case, realistic expectation for in- 
mine use. 



HIGH- AND LOW-TEMPERATURE TREATMENTS 



71° C, 48 Hr. — This treatment was con- 
ducted according to procedures described 
in MIL-STD 8 IOC, Test Method 501.1, March 
10, 1975, except that the oven was 
preheated. 

100° C, 4 Hr . — This treatment was per- 
formed not out of any anticipation of 
similar in-mine conditions, but as a re- 
search probe to study failure modes for 



the apparatus under unrealistically high 
temperatures. If the apparatus were, 
however, not affected by this treatment, 
a higher degree of confidence would be 
gained in their performance ability. A 
convection oven was used in both heat 
treatments. 

-45° C, 16 Hr. — This was arbitrarily 
determined to be a worst case condition. 



RESULTS AND DISCUSSION 



Tread m ill Tes ts 

The results of the treadmill tests 
(fig. 7) are presented in table 3. In- 
cluded are the test subject codes, the 
duration of the tests, and the failure 
modes for each test. Because control 
test durations varied considerably owing 
to the different physiological demands 



placed on the SCSR's, control durations 
were normalized to unity for the sake of 
analysis and comparison. Also, owing to 
scheduling problems, cases arose where 
one test subject ran a treated unit in 
place of another subject. Again, normal- 
ization takes this into account. Failure 
modes were high CO2 and low bag volume. 



TABLE 3. - Treadmill test results 



Draeger 


MSA 


Subject 


Duration, 
min 


Failure mode 


Subject 


Duration, 
min 


Failure mode 



A, 
B. 
C. 



CONTROL 



118 
66 
64.5 



Low bag volume. 
High CO2. 
Do. 



119 
63 
68.5 



High CO2. 
Do. 
Do. 







71° C, 


48 HR 






B 


66 
66 
67 


Exceeded control. 
Do. 
Do. 


A 


120 
63 
69.5 


Exceeded control. 


C 


B 


Do. 


C 


C 


Do. 



71 



^64 
(2) 



100° C, 4 HR 



Candle failure; 
high CO2 ; low O2 . 
Do. 
Do. 



63 

63 
69.5 



Exceeded control. 

Do. 
Do. 







-45° C, 


16 HR 






A 


118 
66 

64.5 


Exceeded control. 
Do. 
Do. 


A 


119 
63 
65 


Exceeded control. 


B 


B 


Do. 


C 


c 


High CO2. 







SHOCK AND VIBRATION 



120 
64.5 
64.5 



Exceeded control. 
High CO2. 
Exceeded control. 



^Technical failure at start, ^^^fg support failure. 



120 
62 
63 



Exceeded control. 
High CO2. 
Do. 



27 




FIGURE 7. - Treadmill testing. 



28 



The most severe treatment for the Drae- 
ger unit was heating to 100° C for 4 hr. 
Inspection of the apparatus showed bulg- 
ing of the plastic case (fig. 8). An 
average volumetric increase of approxi- 
mately 4% was measured. Other noted 
changes were cracked and warped lenses on 
goggles in some cases (fig. 9) and warp- 
ing of the three inhalation valves at the 
breathing bag-breathing tube interface 
(fig. 10). During treadmill testing, it 
was apparent that the chlorate candles 
were not working. Manual startings of 
the apparatus were necessary in all 
cases. The three inhalation valves, 
which were warped, permitted exhaled gas 
to flow into the breathing bag where we 
measured gas concentration continuously. 
All the Draeger tests for this treatment 
were considered technical failures owing 
to candle failure, sporadic high CO2 , and 
initially low oxygen. Two test subjects 



were able to start their units manually 
and finish man-test 4; a third was unable 
to continue owing to lightheadedness. 
The third subject also experienced O2 
concentrations of as low as 15.1% before 
the KO2 bed started. While the tests 
were technically failures, they would 
have successfully protected a person in 
an irrespirable atmosphere in two cases 
and with some physiological side effects 
in the third case. Cold treatment, shock 
and vibration, and heating at 71° C did 
not affect the Draegers to any signifi- 
cant degree. 

The most severe treatment for the MSA 
unit was the shock and vibration. A 
coughing problem was noted in both con- 
trol and treated apparatus and became 
more severe for the vibrated units. This 
led to taking outside breaths and exhal- 
ing through the unit to clear it of 





•WWaMK**!*/!" 






■*'y-.!-'^-.*40 cit*' 




FIGURE 8. - Draeger unit after 100° C, 4 hr. 



29 





FIGURE 9. - Draeger goggles after 100° C, 4 hr. 



FIGURE 10«- Draeger inhalation valves after 100 C, 4hr. 



what we believe to have beea KO2 dust. 
Coughing occurred upon initial donning in 
all cases and any time the apparatus was 
jostled during the first 5 tain. Suspect- 
ing KO2 dust as the cause, we discon- 
nected the bag-hose assembly from the KO2 
canister of a new MSA OSR. Inhaling from 
the bags and hose did not cause coughing, 
whereas inhaling from the KO2 canister 
directly did cause coughing. A more 
effective filter on the KO2 bed would 
easily solve this problem. 

Coughing was experienced in some tests 
of the MSA units previously at NIOSH and 
in the MSHA field evaluation, but not to 
the degree that we have experienced it. 
We postulate that those test subjects who 
have not been subjected to a dusty coal 
mine environment are more likely to ex- 
perience coughing since their lungs would 
have more sensitivity to irritating par- 
ticulates. We conclude that while the 
coughing necessitated outside breaths in 
some cases, the problem was not serious 
enough to consider the tests in which it 
occurred failures; this problem will be 
remedied by MSA in production models of 
the units. 

The shock portion of the treatment 
apparently caused the canister assembly 
portion of several of the MSA apparatus 
to become disconnected from the frame 
onto which they were secured with rubber 
shock mounts. It was necessary to tie 



the canister assemblies to the frame with 
wire in some cases. The heat and cold 
treatments did not affect the MSA's to 
any significant degree. 

BMS Tests 

The results of the simulator tests are 
presented in table 4. The normalized 
test times for both treadmill and simu- 
lator tests are given in table 5. Dura- 
tions and failure modes are given. We 
discarded one vibrated MSA unit that 
accidentally was vibrated more than the 
others. Temperature and breathing resis- 
tance limits as defined by NIOSH (maximum 
inhaled temperature, 46° C; maximum peak- 
to-peak resistance at a 120-lpm flow 
rate, 100 mm H2O) were exceeded in some 
cases, but were not used as termination 
factors since NIOSH standards apply 
specifically to NIOSH testing, which is 
different from the simulator test. Fail- 
ure modes included low bag volume, high 
CO2 , and low O2 . As with the treadmill 
testing, the two Draeger units heated to 
100° C suffered candle failure and had to 
be manually started. They were both 
technical and life support failures owing 
to candle failure, low oxygen (12%), and 
high CO2 (5.8%), all of which occurred at 
the start of the test. The other treat- 
ments did not affect the Draegers to any 
significant degree. None of the treat- 
ments affected the MSA's to any signifi- 
cant degree. 



30 



TABLE 4. - Simulator test results 



Draeger 


MSA 


Duration, min Failure mode 


Duration, min Failure mode 



CONTROL 



52.., 
59.., 
76.., 
61.5, 
58.., 



Low bag volume. 

Do. 
Low O2 . 
Low bag volume. 

Do. 



82.5, 
75.., 
61.., 
76.., 
53... 



High CO2. 

Do. 
Low bag volume. 
High O2. 

Do. 



71° C, 48 HR 



65.., 
54.5, 



High CO2. 

Low bag volume. 



68.5, 
79.., 



High CO2 
Do. 



100° C, 4 HR 



0). 
0). 



Candle failure; 
high CO2 ; low O2 
Do. 



65. 
57. 



High CO2 
Do. 



-45° C, 16 HR 



60. 
64. 



Low bag volume. 
Do. 



61.., 
74.5. 



High CO2 
Do. 



SHOCK AND VIBRATION 



58. 
66. 
60. 




Low bag volume, 
High CO2. 



Life support failure. 



TABLE 5. - Normalized test times 



Treatment 



Draeger MSA 



TREADMILL TESTS 



71° C, 48 hr.. 
100° C, 4 hr.. 
-45° C, 16 hr. 



Shock and vibration. 




SIMULATOR TESTS 






71° C, 48 hr 


1.06 
.89 
(2) 
(2) 
.98 

1.04 
.95 

1.08 
.98 


0.99 


100° C, 4 hr 


1.14 
.94 


-45° C, 16 hr 


.82 
.88 


Shock and vibration 


1.07 
1.12 




1.06 



Technical failure. Life support failure at start. 



31 



1.25 



1.15 



1.05 - 



en 

LlI 



Q 
UJ 
Nl 



.95 - 



^ .85 
o 



.75 



o 


1 1 1 
KEY 






- 


' X Treadmill test 










O Simulator test 














CO 




1 






LU 


- 








5 








2 


^ 


H 




o 








t- 


o 


X 

X 
X 




o i 

/-S points 3 

Wx ; 

o < 


\ 


LlI 

(- 

Q 


8 






[ 


N 
_l 




± 1 standard deviation 




< 












o 






o 












z 


o- 




' — — 











Tech 


nical - 






1 


fail 

1 


ure 

1 






Control 


71 100 -45 Shock 






TEMPERATURE 


:,°c 


vibr 


3tion 





.65 



FIGURE IL - Normalized treadmill and BMS 
test data, Draeger. 

Figures 11 and 12 show plotted results 
of the treadmill and BMS tests for the 
Draeger and MSA. SCSR's, respectively, in 
normalized form with treadmill control 
tests having a value of 1.00. Only the 
100° C treatment on the Draegers had much 



.dO 


1 


1 1 KEY 1 






o 


X Treadmill test 
O Simulator test 


- 


1.15 


- 


O O 


o 


.05 


o 
o 


o 


X 


.95 


2 points-''^ 

~ ± 1 standard deviation o ^ 




O 


o 


" 


.85 


r li 


- 


-7c; 


o 


1 1 1 


- 



Control 71 100 

TEMPERATURE, °C 



-45 



Shock 
vibration 



FIGURE 12. - Normalized treadmill and BMS 
test data, MSA. 

effect on duration of the units. For 
comparison, the control test durations of 
the simulator tests were averaged and 
standard deviations computed, normalized 
with respect to the average, and plotted. 
This shows that the variation of the 
treated units compared with that of the 
untreated units is similar. 



CONCLUSIONS 



The results of this study show that the 
Draeger OXY-SR 60B and the MSA 60-min SSR 
chemical oxygen self-rescuers approved by 
NIOSH will successfully withstand the 
mining environment in the areas of 
temperature extremes and physical abuse 
likely to be encountered when the appara- 
tus are either mounted on mining 
machines, carried, or transported. 

The Draeger OXY-SR 60B experienced can- 
dle failure when heated to 100° C for 



4 hr. It is recommended that the unit 
be kept below its approved maximum 
storage temperature (70° C). The MSA 60- 
min self-rescuer evidenced problems with 
coughing upon initial donning with most 
units, treated and untreated. This prob- 
lem was magnified when the apparatus 
was vibrated and shocked. A simple 
modification of the exit filter in the 
KO2 canister can be made to increase its 
effectiveness, and MSA plans to make this 
modification in production models. 



32 



CHEMICAL OXYGEN SELF-CONTAINED SELF-RESCUER ESCAPE STUDY 

By John G. Kovac,! D. Randolph Berry, 2 Diane M. Doyle, 3 
Elizor Kamon,4 and Donald W. Mitchell^ 



ABSTRACT 



An underground escape study evaluating 
the performance of chemical oxygen self- 
contained self -rescuers (SCSR's) was con- 
ducted by the Bureau of Mines. Six vol- 
unteer coal miners, ranging in age from 
24 to 61, were the test subjects. All 
six subjects traveled along a special 
escapeway which was 7,825 ft long and had 
seam heights ranging from 30 in to 7 ft. 
Average escape speeds ranged from 



96 ft/min crawling to 264 ft/min for 
head-bent walking. Average life of the 
chemical oxygen SCSR's was 60.8 min. The 
total distance traveled before the ex- 
haustion of a chemical oxygen SCSR was 
independent of travel speed. The average 
breathing rate was 41 1/min, and 
the average oxygen consumption was 
1.38 1/min. 



INTRODUCTION 



In May 1981, the Bureau of Mines 
conducted an underground escape study on 
the performance of chemical oxygen 
SCSR's. Specifically, the purpose of 
this study was to obtain detailed, 
quantitative information in the following 
areas: 

1. Escape speeds in different mine 
conditions. 

2. Evaluation of chemical oxygen 
SCSR's in actual escape conditions. 

3. Miner physiology in actual escape 
conditions. 

The Draeger OXY SR 60B and the MSA 60- 
min SSR were the only SCSR's evaluated in 
this study. S Both emergency breathing 
apparatus are chemical oxygen SCSR's, 

'Supervisory mechanical engineer, 
Pittsburgh Research Center, Bureau of 
Mines, Pittsburgh, Pa. 

^Technical staff consultant, Foster- 
Miller Associates, Inc., Waltham, Mass. 

-^Pittsburgh Division staff engineer, 
Foster-Miller Associates, Inc., Waltham, 
Mass. 



using potassium superoxide to generate 
oxygen and remove carbon dioxide. Fig- 
ures 1 and 2 show the two SCSR's as worn 
by a miner. The size and weight of these 
two units are given in table 1. Since 
both the design, function, and overall 
performance of the Draeger and MSA SCSR's 
are similar, the results of the escape 
study are reported without identifying 
either apparatus. 

TABLE 1. - Specifications for approved 
1-hr chemical oxygen SCSR's 





Carrying 
wt, lb 


Deployed 
wt, lb 


Volume, 
CU in 


Draeger OXY 
SR 60B 

MSA Model 
60-min SSR. 


8.4 
9.1 


7.4 
6.7 


366 
360 





"^Professor of applied physiology and 
ergonomics, Pennsylvania State Univer- 
sity, University Park, Pa. 

^Pittsburgh Division Manager, Foster- 
Miller Associates, Inc., Waltham, Mass. 

^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



33 





FIGURE 1. - Draeger OXY SR 60B worn by a miner. FIGURE 2. = MSA 60-min SSR worn by a miner. 

TEST PROCEDURE 



The underground escape study was con- 
ducted in the Rochester and Pittsburgh 
Coal Co.'s Emilie No. 4 Mine. The escape 
route is shown schematically in figure 3. 
This route was not a designated escapeway 
for the mine. Instead, the route of 
travel was specially selected to meet the 
following requirements: 

1. Must be at least 1 hr long for the 
fastest miner. 

2. Different segments of the route 
must have different seam heights. 

3. Different segments of the route 
must have different ground conditions 
such as wet and dry, level and sloping, 
and smooth and irregular roof and floor. 

As shown in figure 3, the escape route 
was divided into seven segments according 
to the travel height in each segment. 
The total length of the escape route was 
7,825 ft with seam heights ranging from 
30 to 78 in. Escapeway conditions in- 
cluded such factors as irregular roof. 



loose material on the floor, water, and 
an uneven or pitched roof. To complete 
the escape route, test subjects were 
required to crawl, duckwalk, and walk 
upright. 



Employees 
burgh Coal 



Test Participant s 

of the Rochester 
Co. volunteered 



and Pitts- 
for this 



study. The six men and their job classi- 
fications are listed in table 2. 



TABLE 2. - Test participants 









Underground 


Miner 


Age 


Job description 


experience, 








Y^ 


A • • • • 


61 


Superintendent- 
mine foreman. 


41 


B 


54 


General assistant. 


35 


C 


49 


Section foreman... 


6 


D 


49 


Shift foreman 


10 


E 


34 


Safety inspector.. 


10 


F 


24 


Maintenance 


6 






foreman. 





34 




HEIGHT OF 
ENTRY, in. 



Full walk, 
head bent 



Duckwalk 



Full walk, 

head bent 
Duckwalk, Full walk up 

obstacles 10% grade 



FIGURE 3, - Escape route. 



The test participants were chosen from 
company management' to represent a broad 
cross section of the mine population in 
good health. For legal and ethical rea- 
sons, it was not possible to involve in 
these tests persons having respiratory, 
circulatory, coronary, or ambulatory 
deficiencies. 

Before any underground testing, the six 
volunteers were trained in the use of 
SCSR's, including how to recognize when 
an SCSR becomes depleted. Also, the six 
volunteers were given a thorough stress 
test by a physician. 

The purpose of the stress test was to 
measure the physiological response of 

'A nationwide coal strike precluded the 
involvement of union miners during the 
period that these tests were conducted. 



each volunteer to physical exercise. The 
stress test involved step increases in 
uphill walking either up to exhaustion, 
or until the supervising physician found 
it necessary to terminate the test. 
Towards the end of the test, at the peak 
of exertion, the test subjects expired 
air was collected for the measurement of 
maximum rate oxygen uptake (Vo„ max)» 
which is also called maximum aerobic 
capacity. Since the cardiograms were 
continuously monitored during the stress 
test, the maximum heart rate (HR^ax) ^t 
Vqo max was also available. Other data 
was also collected including age, weight, 
and height as well as the resting^ and 
maximum values of ventilation ^rate (VE), 
rate of oxygen uptake (Vq,, )> and 
heart rate (HR) for each tested miner. 
All of this information is shown in 
table 3. 



TABLE 3. - Age, physical characteristics, resting and maximum heart rates 
(HR, beats/min) , minute O2 uptake (Vo2)> and pulmonary ventilation (Vg) 
for each miner 



35 





Age, 


Ht, 


Wt, 




Resting 




Maximal 


Miner 


HR, 
beats/ 
mi n 


V02 


1/min 


HR, 
beats/ 
nn'n 


V02 


Ve, 
1/min 




yr 


cm 


kg 


1/min 


ml-kg/min 


1/min 


ml-kg/min 


A • • • • 


61 


185 


71.2 


76 


0.28 


3.93 


10 


156 


1.60 


22.5 


33 


B • • • • 


54 


160 


77.6 


69 


.37 


4.77 


14 


155 


2.39 


30.8 


64 


• • • • 


49 


166 


78.0 


72 


.37 


4.74 


12 


168 


2.87 


36.8 


54 


D. ... 


49 


172 


80.7 


84 


.39 


4.83 


ND 


200 


^2.70 


'33.5 


ND 


£ . . . . 


34 


166 


81.3 


96 


.36 


4.43 


12 


200 


3.17 


38.9 


97 


F . . . . 


24 


183 


82.9 


74 


.30 


3.62 


ND 


182 


3.48 


42.0 


86 


^ • . . . 


45.2 


172 


78.6 


78.50 


.35 


4.39 


12 


176.8 


2.47 


34.1 


62.3 


SD... 


13.6 


10 


4.1 


9.95 


.04 


.50 


1.63 


20.4 


0.88 


10.2 


25.5 



ND No data. 

1 



Estimated by extrapolation to the HRmax from Vo„ -HR relationship during the escape. 



Test Sequence 

Each of the six miners traveled 
the escape route once per day for 4 con- 
secutive days. The first day was a prac- 
tice run for preliminary observations 
and to acquaint each man with the 
route. On each day of the following 
3 days of time trials, two miners 
traveled the escape route without wearing 
an SCSR, two miners escaped wearing 
SCSR's, and two miners escaped wearing a 
recording respiration meter with face 
mask, called an Oxylog, which is shown 
in figure 4. These assignments were 
rotated daily so that after 3 days each 
miner had escaped without an SCSR, 
wearing an SCSR, and wearing the 
Oxylog, as shown in table 4. The 
miners' instructions were to travel as 



fast as possible, yet 
trial. 



complete the 



TABLE 4. - Test sequence 



Miner 


Day 1 


Day 2 


Day 3 


A 


Normal 

Normal 

Oxylog 

SCSR 

SCSR 

Oxylog 


Oxylog 

SCSR 

SCSR 

Oxylog 

Normal 

Normal 


SCSR 


B 


Oxylog 
Normal 


C 


D 


Normal 


E 


Oxylog 
SCSR 


F 



Other mine employees were located at 
stations through 7 to record the times 
of each test subject and to measure and 
record heart rate and blood pressure. 
Each of these measurement personnel 
was a certified emergency medical 
technician. 



36 




FIGURE 4o - Miner wearing an Oxylog, 



At station 0, test subjects were 
started on the route individually at 15- 
min intervals and departure times were 
recorded. The SCSR wearers activated 
their units, and the time was recorded. 
In addition, the miners wearing SCSR's 



recorded the time and location when their 
SCSR's were no longer usable. An SCSR 
was judged to be exhausted when its 
breathing bag became deflated or when 
breathing resistance increased. 



RESULTS 



Escape Speed 

The average travel speed for each seg- 
ment along the escape route is given in 
table 5. Based on these data, figure 5 



was constructed, giving a curve of escape 
speed as a function of escapeway height. 
As expected, the lower the seam height, 
the slower the escape speed. 



37 



TABLE 5. - Average travel speed 



Route 


Mode of 
travel 


Speec 


, ft/min 




segment 


Normal 


With 


With 










SCSR 


Oxylog 


c 


to 1.. 


Head bent 


264 


212 


220 


E 


1 to 2.. 


Duckwalk, 


103 


100 


97 


£ 


2 to 3., 


Crawl. . . . 


96 


85 


79 


ffl 


3 to 4., 


Duckwalk. 


123 


94 


100 


o. 


4 to 5.. 


Duckwalk . 


105 


^85 


86 


tfi 

UJ 


5 to 6.. 


Head bent 


236 


174 


220 


Q. 
< 


6 to 7.. 


Upright. . 


244 


2 234 


217 


u 

(0 


^3 of the 6 SCSR's exhausted 


during 


UJ 


phase. 










^The other 3 SCS 


R's exha 


Lusted 


during 




this phas 


e. 











Wearing an SCSR or the Oxylog had a 
definite influence on travel speed, as 
shown in table 6. 



300 



250 



200 



150 



100 



50 



Each dot represents 
average speed for 
6 miners 



Below average 
owing to 10% 
uphill grade ~ 



± 



-Below average 
owing toground 
condition 

_J I 



30 40 50 60 

ESCAPEWAY HEIGHT, 

FIGURE 5. - Test results. 



70 



80 



TABLE 6. - Travel speed while wearing a 
respiratory device 

Average speed over 



Travel mode 
Normal .............. 


entire route, 
ft/min 

161 


Wearing SCSR 


135 


Wearing Oxylog 


136 



It is not surprising that Oxylog and 
SCSR produced the same decrease in speed 
because the two units weigh about the 
same, are carried on the body in a simi- 
lar manner, and require breathing through 
a mouthpiece or mask. 

This decrease in travel speed while 
wearing a respiratory device was fairly 



consistent in each segment of the travel 
route. 

Life of Chemical Oxygen SCSR's 

Six chemical oxygen SCSR's were tested 
during this study, one by each of the six 
test subjects. The results are sum- 
marized in table 7. 

In general, the faster the miner 
traveled, the sooner the SCSR was con- 
sumed. As can be seen in the last 
column of table 7, chemical oxygen 
SCSR's seem to provide enough oxygen for 
a constant amount of work regardless of 
the speed at which the work is carried 
out. 



TABLE 7. - Life of chemical oxygen SCSR's 



Miner 



A. 
B, 
C, 
D. 

E. 
F. 



Escape time 


Life 


of SCSR 


(travel + wait) , 


Time, 


Distance, 


min 


min 


ft 


70+13 


I77 


6,850 


62+10 


H\ 


6,215 


51+14 


Ho 


6,850 


64+14 


Ho 


5,640 


51+14 


249 


5,700 


50+ 9 


2 58 


7,340 



Distance traveled 
X body weight, 
million ft-lb 



1.07 
1.06 
1.18 
1.00 
1.02 
1.34 



^Includes 2-min wait (after donning SCSR) before starting escape. 
^Includes 1-min wait (after donning SCSR) before starting escape, 



38 



Physiological Data 

The physiological response of five of 
the six miners during escape is samma- 
rized below: 

Average breathing 41 1/min. 
rate. 

Average oxygen 1.38 1/min. 
consumption. 



data for miner F, and 
rates for miners D and F, 



no ventilation 



Average heart 
rate. 

Oxygen consump- 
tion (walking). 



143 beats/min. 



0.35 ml per meter 
traveled per kilogram 
body weight. 



SCSR's used in U.S. underground coal 
mines must be jointly approved by the 
Mine Safety and Health Administration 
(MSEA) and the National Institute for 
Occupational Safety and Health (NIOSH) 
according to 30 CFR 11. The major per- 
formance requirement in 30 CFR 11 is the 
60-min man-test 4. Specific activities 
in man-test 4 include walking, running, 
climbing a vertical treadmill, and carry- 
ing and pulling weights. All of the 
travel speeds and carried weights are 
prescribed for a total of 52 min, inter- 
spersed with 8 min of rest for samplings 
and readings. 



Oxygen consump- 
tion (crawling). 



0.70 ml per meter 
traveled per kilogram 
body weight. 



Instrumentation malfunctions during the 
underground test program resulted in the 
loss of some data, no oxygen consumption 



In this escape speed study, the oxygen 
consumption (1.38 l/mln) was the same and 
the respiration rate (41 1/min) was 30% 
higher than the rates of miners during 
the performance of the corresponding ele- 
ment of man-test 4.8 



SUMMARY 



The main points of this study are sum- 
marized below: 

1. Wearing an SCSR decreased travel 
speed. 

2. The duration of chemical oxygen 
SCSR's ranged from 49 to 77 min with an 
average duration of 60.8 min. 

3. For the escape route in this study, 
the duration of a chemical oxygen SCSR 
(as measured in minutes) seems directly 
related to the speed of the escape, 
whereas the total work effort allowed by 
the SCSR (as measured by miner's body 
weight X distance traveled before the 
SCSR expires) is remarkably constant, 
regardless of the speed with which the 
work effort is performed. 



4. The physiological cost of the 
escape route in this study was greater 
than the physiological cost of man- 
test 4. 

In terms of future research, the Bureau 
of Mines plans to conduct a similar study 
evaluating the performance of commer- 
cially available NIOSH-MSHA-approved 
compressed-oxygen SCSR's. 



Q 

°Kamon, E. , T. Bernard, and R. Stem. 
Steady State Respiratory Responses to 
Tasks Used in Federal Testing of Self- 
Contained Breathing Apparatus. Am. Ind. 
Hygiene Assoc. J., 1975, pp. 885-896. 



39 



MEDIUM FREQUENCY RADIO COMMUNICATION SYSTEM FOR MINE RESCUE 
By Harry Dobroski, Jr^1 and Larry G. Stolarczyk2 

ABSTRACT 



Theoretical and experimental work spon- 
sored by the Bureau of Mines indicated 
that medium frequency (MF) signals 
(300 kHz to 3 MHz) propagate through 
natural media (water, rock, coal, etc.) 
and down the passageways of the under- 
ground mine. Existing passageway con- 
ductors in the "wire plant" such as 
track, wire rope, telephone cable, elec- 
trical power distribution systems, etc., 
cause a low-loss transmission line signal 
propagation mode to exist. This paper 



describes sytem concepts as well as the 
new MF radio equipment that has been 
developed for communications in the un- 
derground mining complex. The new equip- 
ment includes a lightweight vest trans- 
ceiver that is potentially useful for 
rescue personnel to establish emergency 
communication links to the rescue team 
communications center. This paper also 
describes the most recent field test 
results. 



INTRODUCTION 



In the event of a mine disaster such as 
an explosion where miners may be trapped 
underground, efficient rescue efforts are 
essential. Rapid and efficient opera- 
tions not only enhance the possibility of 
achieving a successful rescue, but also 
reduce the risks to the rescue teams. In 
many instances, rescue teams have risked 
their lives in areas where there were no 
trapped miners only to learn later that 
if the rescue effort had been directed 
into other areas of the mine, lives could 
have been saved. 

The traumatic event of a mine disaster 
poses problems that few people compre- 
hend. It is an event that takes place in 
a confined area where toxic gases, oxygen 
deficiency, poor visibility, and the 
danger of a recurring disaster are ever 
present. If miners actually survived the 
initial event, the question exists as to 
their condition and location. Obviously 
survivors are not likely to be found 
where expected because they would move to 

Supervisory electrical engineer, 
Pittsburgh Research Cneter, Bureau of 
Mines, Pittsburgh, Pa. 

^Director of Research, A.R.F. Products, 
Inc., Raton, N. Mex. 



safer locations, seek out alternate es- 
cape routes, or as a last resort, barri- 
cade. In any event, rapid communications 
with, and rescue of, these trapped miners 
is essential. The difference between 
life and death can often be measured in 
minutes. 

The Bureau of Mines has developed 
through-the-earth seismic and electromag- 
netic (EM) location and communication 
systems that enhance escape and rescue to 
a large degree. The first system is 
presently operational and the second is 
still in the research and development 
stage. Nevertheless, even if fully im- 
plemented, there will never be any assur- 
ance that all miners have truly been 
located. Injury or other factors may 
make it impossible to utilize the fea- 
tures of either the seismic or electro- 
magnetic systems. The keystone of any 
rescue effort is, and will remain, the 
rescue team. 

Rescue team communication is second in 
importance only to the life support sys- 
tem. Without it, the rescue effort goes 
slowly, increasing the danger to both the 
team and the trapped miners they hope to 
rescue. 



40 



RADIO PROPAGATION IN MINE ENVIRONMENTS 



Although radio transmission on the sur- 
face of the earth is well understood, 
transmission in an underground environ- 
ment generally is not. Complex inter- 
actions occur between the radio wave and 
the environment. Characteristics of the 
geology (stratified layering, boundary 
effects, conductivity, etc.) and the mine 
complex (entry dimension, conductors, 
electromagnetic interferences, etc.) had 
to be measured and understood before any 
practical mine radio system could be 
built. 

In a confined area such as a mine, a 
radio wave can propagate a useful dis- 
tance only if the environment has the 
necessary electrical and physical proper- 
ties. The "environment" takes into 
account the natural geology and fabri- 
cated perturbations such as the mine com- 
plex itself. As an example, if the wave- 
length (A) of a radio wave is small 
compared with the entry dimensions, a 
waveguide mode of propagation is possi- 
ble. Attenuation depends primarily upon 
the physical properties of the entry such 



as cross sectional area, wall roughness, 
entry tilts, and obstacles in the propa- 
gation path. Secondary effects such as 
the dielectric constants and earth con- 
ductivity also influence attenuation. 

Mine radio systems based upon this 
waveguide effect are available commer- 
cially and have been successfully used by 
rescue teams for short-range coordina- 
tion. These radios operate in the UHF 
band of the radio spectrum and are small 
and convenient to carry and use. Under 
line-of-sight high coal conditions, 
transmission ranges in excess of 300 m 
(1,000 ft) are often possible. However, 
in low coal, or when going around ob- 
stacles and corners, the range is severe- 
ly reduced. Clearly a system is needed 
that would permit long-range radio com- 
munications not only among team members, 
but also with other teams and the surface 
command center. MF radio provides not 
only this capability, but also that of 
communicating with trapped miners from 
within the mine. 



GENERAL IN-MINE MEDIUM FREQUENCY RESULTS 



Considerable research has been con- 
ducted within the last 8 years in the 
area of underground MF transmissions. 
This research showed that MF signals 
could propagate for great distances in 
most geologies and offered the hope of 
a whole-mine radio system. The Bureau 
of Mines and the South African Cham- 
ber of Mines (SACM) pursued research 
independently. 

Around 1974, SACM introduced a new 
single-sideband system and followed up 
later with another designed especially 
for rescue team use. Performance in 
South Africa was reported to be good (13, 
pp. 87-102). 3 The evaluation of theFe 
units in U.S. mines showed them to be 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this paper. 



inadequate. The type of modulation used 
[single sideband (SSB)] made them sensi- 
tive to electromagnetic interference 
(EMI). In addition, power level was far 
too low and inefficiencies in both cir- 
cuit and antenna designs produced short- 
range performance. 

The Bureau's approach to the problem 
was more fundamental. A program was 
designed (1-4, 9-10) and executed to 
study in-mine MF propagation and learn 
how it interacted with the complex envir- 
onment. This environment consists of 
various geological factors such as strat- 
ified layers of different electrical 
parameters, entry size, local conductors, 
EMI, etc. 

Figure 1 is a simplified geometry of 
an in-mine site that illustrates one 
of the most important findings of 



41 




Local mine conductor 




% 



Coal 

or 

entry 



Loop 

transmitting 

antenna 




FIGURE 1. - Coal seam mode. 

the measurement program — the "coal seam 
mode." For this mode to exist, the coal 
seam conductivity (a^) must be several 
orders of magnitude less than that of the 
rock (or-)* A loop antenna that is at 
least partially vertically oriented, pro- 
duces a vertical electric field (E^) and 
horizontal magnetic field (Hx). In the 
rock, the fields diminish exponentially 
in the Z-direction. In the coal seam, 
the fields diminish exponentially at a 
rate determined by the attenuation con- 
stant (a) which in turn depends upon the 
electrical properties of the coal. An 
inverse square-root factor also exists 
because of spreading. The effect is that 
the wave, to a large degree, is trapped 
between the highly conducting rock layers 
and propagates long distances within the 
lower conducting coal seam. The fact 




Signal propagating 
along conductor 



MF signal 
coupling into 
local conductor 



Signal reradiating 
from conductor 



Vest radio concept 



FIGURE 2. - MF parasitic coupling and reradiation. 

that the coal may have entries and cross- 
cuts is of minor consequence. 

In the presence of conductors, the pic- 
ture changes considerably. In this case, 
the effects of these conductors can 
totally dominate over the effects of the 
geology. In general, the presence of 
conductors (rails, trolley lines, phone 
lines) is advantageous. 

MF signals can couple into, and reradi- 
ate from, continuous conductors in such a 
way that these conductors become not only 
the transmission medium but also the 
antenna system for the signals. Figure 2 
illustrates this concept. The most 
favorable frequency depends to some ex- 
tent on the relationship between the 
geology and existing conductors. The 
frequency effects are quite broad. 
Anything from 500 to 800 kHz is usually 
adequate. 



SPECIFIC APPLICATION OF MF COMMUNICATIONS TO ElESCUE TEAMS 



The low attenuation of MF signals in 
many stratified geologies, such as coal 
mines, can be of great benefit to rescue 
teams. If existing mine wiring (like 
powerlines or belt lines) are present, 
the range is even greater. This permits 
a rescue team member to stay in communi- 
cation with other members, the fresh air 
supply, and outside disaster control 
centers. 

To date, MF technology has not been 
specifically applied to rescue team 



communications. Such application is the 
second step in the Bureau's overall MF 
communications program. However, there 
is no basic difference between opera- 
tional MF systems and postdisaster MF 
systems. By October 1982, the Bureau's 
operational MF systems will be in place 
in several cooperating underground mines. 
By October 1983, performance evaluation 
of the systems will be completed. As the 
performance proceeds, emphasis will be 
directed to specific postdisaster-rescue 
applications. 



42 



SYSTEM CONCEPTS 



The main advantage of MF communication 
is simplicity. Figure 3 shows a rescue 
team member equipped with an MF vest ra- 
dio. This vest radio permits rescue team 
members to maintain local communications 
(fig. 4). 

In most cases, rescue teams will uti- 
lize a lifeline for rapid retreat in case 
of smoke when visibility is limited. The 
lifeline offers interesting possibilities 
for MF radio communications. Some rescue 
teams actually use the line already to 
carry communications via sound-powered 
telephones. Such a scheme is both archa- 
ic and ineffective. 

Since this line is a continuous con- 
ductor back to the fresh air base, it 
provides a convenient parasitic path for 
MF communication as shown in figure 5. 
To assure even more reliable communica- 
tions, physical audio links could be made 
with the lifeline as shown in figure 6. 
Such an approach provides redundancy via 
simultaneous audio and radio links. 

Figure 7 illustrates a total MF base 
station for rescue team use. At the 



fresh air base, the briefing officer (as 
the individual is sometimes called) is 
equipped with a standard intrinsically 
safe base station or repeater; the offi- 
cer could also be equipped with a vest. 
With such an arrangement, communications 
are possible not only between rescue team 
members, but also with the surface and 
with other distant rescue teams. In 
addition, it also provides a possible 
link to the trapped miners. 

Since existing mine wiring is extensive 
and minewide, it is easily seen that it 
provides yet another redundant link for 
the rescue team members. Since other 
rescue teams are also in the vicinity of 
mine wiring, interteam communications are 
possible if desired. This concept of 
interteam communications is a radical 
departure from existing procedures. It 
will permit one rescue team, in one part 
of the mine, to modify the ventilation in 
such a manner that it does not degrade 
the ventilation in the vicinity of 
another rescue team. Equally important 
is the fact that trapped miners are also 
probably in the vicinity of existing mine 
wiring . 



LOCATION AND COMMUNICATIONS SYSTEMS FOR THE RESCUE OF TRAPPED MINERS 



So far this paper has primarily ad- 
dressed the application of MF communica- 
tion to rescue teams. However, the ulti- 
mate objective of the rescue operation is 
to reach trapped miners in a timely man- 
ner before they succumb to the effects of 
injury, exposure, or toxic atmospheres. 
To this end, rescue team communications 
is but a part. The key to successful 
rescue lies in the rapid location of the 
trapped miners. Without this, valuable 
time can be wasted in diverting rescue 
efforts to the wrong area, often with 
tragic results. 

Bureau research in the area of location 
has been addressed by through-the-earth 
seismic and EM systems. In the seismic 



system (5, 11-12), trapped miners pound 
on the roof or ribs of the mine to gener- 
ate seismic vibrations. These vibrations 
travel through the overburden to the sur- 
face where they can be detected by sensi- 
tive transducers called geophones. Com- 
puter analysis of the arrival times of 
the seismic signals at the various geo- 
phones permits the source to be accurate- 
ly located. This system is operational 
and is kept in readiness by MSHA Mine 
Emergency Operations. Present Bureau 
research in EM means to locate and com- 
municate with trapped miners is shown in 
figure 8. The system consists of two 
parts, a transceiver that is normally 
carried on the miner's belt and a surface 
system for detection and communications. 



43 




FIGURES, 
vest radio. 



Rescue team member with MF 



In operation, the trapped miner removes 
the transceiver from the belt, deploys a 
self-contained loop antenna, and attaches 
the transceiver to a special cap lamp 
battery. This antenna consists of 90 m 
(300 ft) of No. 18 wire that must be de- 
ployed in the largest area possible to be 
effective. A location signal is trans- 
mitted directly through the earth. 




FIGURE 4. - Basic MF communications among 
rescue team members. 



Lifeline and/or local mine conductors 



Vest 
radio 



Vest |_y 
radio 



Advancing 
rescue team 



Fresh 
air base 



FIGURE 5. - Lifeline as a parasitic MF path. 




Audio plus 
MF 



y 



Vest 
radio 



^ilTcue'^ Local radio 
team communications 



Fresh air 
base 



FIGURE 6. - Lifeline as a redundant communi- 
cations line for MF and audio communication. 

On the surface, sensitive receivers 
detect the signal and locate the source. 
Once detection and location are made, a 
large surface transmitter is deployed 
above the trapped miner. This trans- 
mitter is powerful enough to send voice 
messages by radio, directly down through 
the earth. 



44 



Local mine wiring 




Lifeline 



Base 
station or 
repeater 



Signal coupler 



Microphone 



FIGURE 7. - Total MF base station for rescue 
teams. 



Loop antennas 



Microphone 




Location signal 



Transceiver 



Transmitter 




Battery with 
power (f 
take of f^;^ 



FIGURE 8. = Voice frequency electromagnetic 
system for location and communication with 
trapped miners. 

The trapped talner's transceiver re- 
ceives this voice. The surface personnel 
then ask the miner "yes-no" questions 
concerning his or her condition and that 
of the mine. The miner responds by sim- 
ple on-off keying of the transceiver. In 
this manner a two-way communications link 
is established, entirely through the 
earth, and rescue operations can start in 
the most efficient manner. 

Details of this EM system can be 
found in numerous reports (6-8, 13-15). 
This is known as a voice frequency 
(VF) system because all communications 



take place in the VF band of 300 to 
3,000 Hz. 

The seismic system is very effective in 
mines up to 700 m (2,200 ft) deep, and 
does not require the miner to be equipped 
with any special devices. However, it 
does require the miner to be able to 
pound. Injury or lack of a sufficiently 
heavy object with which to pound may 
render the system ineffective. The most 
serious drawback is that of time. The 
surface receiver station (geophones, 
field truck with computer, etc.) may take 
too long to set up. Bad weather and ter- 
rain can further delay the surface sta- 
tion deployment. 

The EM-VF receiver system is less 
affected by adverse conditions on the 
surface because it is lighter and more 
easily transportable. However, it has 
its own disadvantages. The trapped miner 
must be equipped with a special trans- 
ceiver, and must be able to deploy the 
antenna in a sufficiently large area. 
Injury or confined quarters may prevent 
deployment. In addition, under the best 
of conditions, the system has a range 
limit of about 300 m (1,000 ft). Al- 
though a new system is under development 
that will increase the range to 1,000 m 
(3,000 ft) (3), this improvement comes 
about only with complex, slowly deployed 
surface equipment. Therefore, it will be 
subject to the same delays as the seismic 
system. 

MF coimnunication offers advantages over 
through-the-earth approaches by permit- 
ting in-mine communications to the 
trapped miners. This could be in addi- 
tion to, or in place of, through-the- 
earth schemes that may fail because of 
excessive overburden or the inability of 
the trapped miner to deploy his or her 
end of the system successfully. Figure 9 
illustrates this concept. 



45 



( ac power line) 
Local conductor 2 




[Rescue team] 



•- [Trapped miner] 



FIGURE 9. - MF in-mine location and commu- 
nication system. 



In this illustration, the trapped miner 
is equipped with a small MF transceiver 
built into the top of the cap lamp bat- 
tery or worn on the belt. Note that this 
is exactly the same packaging concept 
used for the VF through-the-earth system 
shown in figure 8. The intent, however, 
is not to send a signal through the 
earth, but rather to induce a signal onto 
local mine wiring. If this is accom- 
plished, the in-mine rescue team also is 
likely to be in the vicinity of mine wir- 
ing and can receive the signal. It must 
be pointed out very clearly that mine 
wiring does not mean that one continuous 
assembly of wiring is involved. If the 
trapped miner is near a power cable and 
not near a trolley line, and the rescue 
team is near a trolley line and not near 
a power cable, this does not mean that a 
communications link between the two can- 
not exist. An induced MF signal on one 
type of conductor will parasitically 



couple to all others, even if there is no 
physical connection. This is the unique- 
ness of MF communication. 

In operation, the trapped miner would 
deploy an MF loop antenna or coupler, 
preferably onto available local wiring. 
The coupler could be a small device of 
small volume similar to a current trans- 
former. The loop could be a coupler that 
was unwound. In either case, the antenna 
is small. If nearby wiring does not 
exist, the loop could be deployed in 
hope of coupling to distant wiring. 
When so deployed, the transmitter sends 
out MF signals of narrow bandwidth that 
parasitically couple onto mine wiring, 
and are widely distributed. This can 
be received by the in-mine rescue team. 
If this occurs, they will use their 
more powerful MF equipment (vests or base 
stations) to establish a voice link 
to the trapped miner. By asking the 
trapped miner yes or no questions, his 
or her location can be learned. However, 
direct location via MF communication is 
impossible. The parasitic coupling 
characteristics of MF signals do not per- 
mit the through-the-earth VF type of lo- 
cation; the signal could be on many 
conductors. 

Obviously VF and MF systems could be 
combined such that the benefits of both 
VF (fig. 8) and MF (fig. 9) could be ob- 
tained. Equally important is the fact 
that the MF trapped miner device could be 
used in nonemergency situations as a page 
receiver and thereby be a cost effective 
addition to a general mine communication 
system. Table 1 lists MF communication 
system specifications. 



46 



TABLE 1. - MF communication system specifications 

Emissions, narrowband FM: 

Occupied bandwidth kHz, , 10 

Rf frequency kHz.. 60-1,000 

Peak deviation kHz.. ±2.5 

Modulated frequency Hz . . 200-2 ,500 

Receiver, superheterodyne: 

Sensitivity 1.0 pV (12-db sined) 

Selectivity 8-pole crystal filter 

IF 3-db bandwidth (minimum) kHz.. 12 

IF 70-db bandwidth (maximum) ... .kHz. . 22 

RF bandwidth kHz.. 60-1,000 

Squelch Noise operated and tone 

Transmitter, push-pull, class B: 
Output power, W: 

Vest 4.0 

Vehicular 20.0 

Antenna magnetic moment (ATm^): 

Vest 2.1 

Vehicular 6.3 

RF line coupler, transfer impedance (Zj): 
1-in coupler, ohms: 

350 kHz 10.0 

520 kHz 11.2 

820 kHz 17.8 

4-in coupler, ohms: 

520 kHz 10.6 

PERFORMANCE DATA 



In order to evaluate the potential of 
MF signals as a means to locate and com- 
municate with trapped miners, and to pro- 
vide communications for the actual rescue 
team operation, a test was conducted at 
the York Canyon Mine near Raton, N. Mex. , 
in June 1982. This mine is a coal mine 
located in the York seam of the Raton 
Basin, The terrain is hilly such that 
tlie mine overburden varies from about 150 
to 300 m (200 to 800 ft). 

The mine has four main drift entries 
that are about 2,500 m (7,000 ft) long. 
Off these entries, submains were driven 
and longwall mining occurs. A borehole 
is located at about the 2,500 m 
(7,000 ft) mark. This borehole contains 
a twisted pair cable that is associated 
with the fire monitoring system on the 
longwall panels. 

This is an ac mine that transports the 
coal by belt. Rubber-tired vehicles 



provide transportation for personnel and 
supplies. The distance from the portal, 
down the main entries to the longwall 
faces, can be nearly 5,500 m (15,000 ft). 

At the mine portal, a MF signal coupler 
was attached to the mine telephone lines. 
This coupler was controlled by a standard 
MF base station. A second coupler and 
base station were placed at the top of 
the borehole. The coupler was clamped 
around the cable that went down the 
borehole. 

Two personnel entered the mine and, by 
vehicle, traveled down the main entries 
to the vicinity of the borehole [2,500 m 
(7,000 ft)]. These personnel were 
equipped with MF vest transceivers that 
had a magnetic moment of 2.1 ATm^ and a 
sensitivity of 1 V at 52 kHz. The intent 
of the test was to ascertain whether or 
not these personnel could communicate 
with the base at the portal, or the base 



47 



at the top of the borehole. If so, it 
would demonstrate that MF-equipped rescue 
teams could communicate with the outside 
command center without deploying their 
own communications line, or relying on 
the integrity of the mine phone line that 
may, or may not, be intact. In addition, 
it would demonstrate that if a trapped 
miner was equipped with a MF transceiver 
of similar specifications, he or she 
could directly communicate with rescue 
teams in the mine, or search crews on the 
surface who were monitoring any con- 
ductors egressing the mine. 

The result of tVie test showed that com- 
munications were possible from almost 
anywhere in the haulage and belt entries 
to either base station. It was even 



possible for the base at the portal, on 
the telephone line, to communicate with 
the base atop the borehole, on the fire 
monitor line, even though there was no 
physical connection between the two. 
Whenever a vest was within a few feet of 
mine conductors , there was an obvious im- 
provement in clarity and signal strength. 

Although this test was preliminary, it 
clearly highlights the potential of using 
MF communications for search and rescue 
operations. Much more work is necessary 
to measure range from mine wiring when- 
ever the mine is not operating as would 
be the case during search and rescue 
operations. An operational mine produces 
considerable levels of acoustic and EM 
noise which reduces MF system range. 



CONCLUSIONS 



The Bureau of Mines has developed a 
whole-mine MF communication system con- 
sisting of vest transceivers, base sta- 
tions, and repeaters. The primary use of 
the system is for operational mine com- 
munications via parasitic coupling onto 
existing mine conductors. The system is 
directly applicable to rescue team and 
trapped miner communications. 

When applied to a rescue scenario, res- 
cue team members can maintain local com- 
munications and communications with the 
fresh air base. Communications with 
other rescue teams and with the sur- 
face operations-command center is also 
possible, 

A test was conducted at the York Canyon 
Mine (New Mexico) that demonstrated the 
potential of MF communications in the 
location, search, and rescue scenario. 
In this test, simulated trapped miners 



and rescue team personnel were able to 
communicate with two outside base sta- 
tions that were monitoring signals 
coupled onto mine wiring that egressed 
the mine. 

Because rescue team members are 
equipped with life support hardware, the 
existing vest concept will have to be 
modified to account for this. The 
present physical configuration of the 
vest is in conflict with the physical 
configuration of the life support system. 
This, however, is a minor problem. 

Transceivers will have to be developed 
for mining personnel to have on their 
persons for emergency use. Such a device 
would be functionally similar to the 
vests. A convenient packaging arrange- 
ment would be as part of the cap lamp 
battery. 



48 



REFERENCES 



1. Cory, T. S. Electromagnetic Propa- 
gation in Low Coal Mines of Medium Fre- 
quencies (Contract H0377053, Rockwell 
Internat.). BuMines OFR 63-82, June 12, 
1978, 96 pp.; NTIS PB 82-202656. 

2. - Propagation of EM Signal in 
Underground Metal/Nonmetal Mines (Con- 
tract J0308012). July 1980; available 
for consultation at Bureau of Mines 
Pittsburgh Research Center, Pittsburgh, 
Pa. 



3. 



Propagation of EM Signals 



in Underground Mines (Contract H0366028, 
Collins Commercial Telecommunications 
Group, Rockwell Internat.). BuMines OFR 
136-78, Aug. 22, 1977, 158 pp.; NTIS PB 
289 757. 

4. Develco, Inc. EM System Deep Mines 
(Contract J0199009). May 1979; available 
for consultation at Bureau of Mines 
Pittsburgh Research Center, Pittsburgh, 
Pa. 

5. Durkin, J., and R. J. Greenfield. 
Evaluation of the Seismic System for Lo- 
cating Trapped Miners, BuMines RI 8567, 
1981, 55 pp. 



9. Lagace, R. L. , M. L. Cohen, A. G. 
Emslie, and R. H. Spencer. Technical 
Services for Mine Communications Re- 
search. Propagation of Radio Waves in 
Coal Mines (Contract H0346045, Arthur D. 
Little, Inc.). BuMines OFR 46-77, Octo- 
ber 1975, 187 pp.; NTIS PB 265 858. 

10. Lagace, R. L. , A. G. Emslie, and 
M, A. Grossman. Modeling and Data Analy- 
sis of 50 to 5000 kHz Radio Wave Propaga- 
tion in Coal Mines. Technical Services 
for Mine Communications Research (Con- 
tract H0346045, Arthur D. Little, Inc.). 
BuMines OFR 83-80, February 1980, 
109 pp.; NTIS PB 80-209455. 

11. Pennsylvania State University. 
Theoretical Investigation of Seismic 
Waves Generated in Coal Mines (Contract 
G0155044). 1975; available for con- 
sultation at Bureau of Mines Pitts- 
burgh Research Center, Pittsburgh, Pa. 

12. Sonic Sciences. Auto Detection 
Algorithm for MSHA's Seismic Location 
System (Contract J0395064) . 1979, 
70 pp.; available for consultation at 
Bureau of Mines Pittsburgh Research Cen- 
ter, Pittsburgh, Pa. 



6. Geyer, R. G. , G. V. Keller, and T. 
Ohya. Research on the Transmission of 
Electromagnetic Signals Between Mine 
Workings and the Surface (Contract 
HO 101691, Colo. School Mines). BuMines 
OFR 61-74, Jan. 10, 1974, 124 pp.; NTIS 
PB 237 852. 



13. Wait, J. R. Electromagnetic 
Guided Waves in Mine Environments. Pro- 
ceedings of a Workshop (Contract 
HO 15 5008, Nat. Telecommunications and 
Inf. Admin., U.S. Dept. Commerce). Bu- 
Mines OFR 134-78, May 31, 1978, 333 pp.; 
NTIS PB 289 742. 



7. Hill, D. A., and J. R. Wait. Ana- 
lytical Investigations of Electromagnetic 
Location Schemes Relevant to Mine Rescue 
(Contract H0122061, Inst. Telecommunica- 
Sci., U. S. Dept. Commerce). BuMines OFR 
25-75, Dec. 2, 1974, 147 pp. 

8. Kehrman, R. F., A. J. Farstad, and 
D. Kalvels. Reliability and Effective- 
ness Analysis of USBM Electromagnetic Lo- 
cation System for Coal Mines, Final Re- 
port (Contract J0166060, Westinghouse 
Electric Corp.). BuMines OFR 47-82, 
Dec. 1, 1978, 153 pp.; NTIS PB 82- 
201385. 



14. Wait, J. R. , and D. A. Hill. Ana- 
lytical Investigation of Electromagnetic 
Fields in Mine Environments (Contract 
H0155088, NOAA, U.S. Dept. Commerce). 
BuMines OFR 53-77, Nov. 15, 1976, 200 pp. 

15. Wait, J. R. , D. A. Hill, and D. B. 
Seidel. Further Analytical Investiga- 
tions of Electromagnetic Fields in Mine 
Environments (Contract HO 15 5008, Inst, 
for Telecommunication Sci., U.S. Dept. 
Commerce). BuMines OFR 86-78, Feb. 1, 
1978, 273 pp.; NTIS P8 284 553. 



49 



FINDING AND COMMUNICATING WITH TRAPPED MINERS 
By S. Shope,1 J. Durkin, 1 and R. Greenfield2 



ABSTRACT 



The Bureau of Mines has performed re- 
search and developmental work on methods 
to locate and communicate with miners 
trapped underground following a mine 
disaster. This work has evolved two ma- 
jor systems to accomplish the objective: 
(a) the seismic system and (b) the elec- 
tromagnetic (EM) system. 

The seismic system detects, at the sur- 
face, vibrations generated by the trapped 
miner pounding on the roof of the mine 
with any implement at his or her dis- 
posal. The vibrations can be used to 
determine the location of the miner. 
Tests have shown that the system is 
effective in locating the miner at depths 
to 2,000 ft. The seismic system is 



presently operational and maintained in a 
state of readiness by the Mine Safety and 
Health Administration (MSHA). 

The EM system makes use of a belt-worn 
radio-type transmitter that the miner 
activates when trapped. The signals from 
the transmitter are sent to the surface 
where surface personnel can detect them 
and locate the miner's position. Once 
location is determined, surface personnel 
can transmit voice signals to the miner 
to establish communication. Tests have 
shown the system to be effective in lo- 
cating the miner at depths to 1,000 ft. 
Studies are continuing to improve the 
performance of the system. 



INTRODUCTION 



Mine disasters continue to have a major 
impact on underground mining from both an 
economic and psychological perspective. 
Disasters are usually caused by explo- 
sions, fires, cave-ins, or floods. The 
time period immediately following a dis- 
aster and up to the point when the mine 
is again secured is the focus of the Bu- 
reau's postdisaster program. Some disas- 
ters are so violent and widespread that 
they immediately kill all underground 
personnel; however, it is not uncommon 
for a disaster to be confined to a small 
underground locale. Even small confined 
events have the potential to so disrupt 
the mine that aftereffects can contribute 
greatly to the death toll. A prime exam- 
ple of this is disasters caused by explo- 
sions. The miners not immediately killed 

'Electrical engineer, Pittsburgh Re- 
search Center, Bureau of Mines, Pitts- 
burgh, Pa. 

^Professor of geophysics. Department of 
Geosciences, Pennsylvania State Univ., 
University Park, Pa. 



by the fire and blast have the potential 
of succumbing to toxic gases produced by 
the explosion. Studies have shown that 
miners that do not have access to immedi- 
ate evacuation stand the best chance for 
survival if they barricade themselves. 
Barricading limits the amount of poison- 
ous gas that the trapped miners are ex- 
posed to. However, once barricaded, the 
miners cannot leave until rescued and may 
be considered prisoners of the mine. 
Usual means of communication may be de- 
stroyed, prohibiting members of the res- 
cue team from communicating with the 
trapped personnel. Without this communi- 
cation, the rescue team knows little 
about the number, condition, or location 
of the barricaded miners. The last fac- 
tor is regretable, since reliable knowl- 
edge on the location of the entombed min- 
ers could lead to the prompt arrival of 
the rescue team and could prevent un- 
necessary deaths. In addition, the res- 
cue team itself would be exposed to less 
hazard by knowing directly where to 
search. 



50 



Following the 1968 Farmington Mine dis- 
aster, the Bureau contracted the National 
Academy of Engineering (NAE) O)^ to 
reconimend means to increase the probabil- 
ity of survival and rescue of miners in 
mine disasters. The report recommended 
that the Bureau develop new communication 
techniques to detect and locate trapped 
miners. The Bureau considered these 
recommendations as the starting point 
for a continuing concentrated research 
effort to improve survival and rescue 
capability. 

The condition following a mine disaster 
is unpredictable; cables may be severed 
and passageways blocked. A hardened 



communication system that advances with 
the face would be prohibitively expensive 
and could not be considered 100 pet reli- 
able. It became apparent that the best 
approach would be a technique that would 
allow communication directly through the 
mine workings or overburden strata. 

Two major areas of detection and loca- 
tion communications were recommended by 
NAE and continue to be investigated: (a) 
seismic and (b) electromagnetic (EM). 
This paper describes the concept of both 
techniques, the present status of each, 
and the program plans for future research 
in these two areas of postdisaster com- 
munication and location. 



THE SEISMIC LOCATION SYSTEM 



In the 1970 NAE report, it was sug- 
gested that a seismic technique might be 
capable of detecting and locating trapped 
miners. It was proposed that the miner 
would strike a part of the mine with any 
heavy object that could be found. The 
resulting vibrations would then be 
detected on the surface by the use 
of seismic transducers (seismometers) 
which will be referred to as geophones. 
The geophones convert seismic signals 
to voltages that are then ampli- 
fied, filtered, and recorded. By com- 
paring the relative arrival times at 
several geophone locations, the trapped 
miner's location can be readily deter- 
mined. This concept may be visulaized in 
figure 1. 



^ 

-'Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this paper. 



In 1971 Westinghouse Electric Corp. 
( 23 ) built and tested such a system. 
From 1972 until the present. Westing- 
house, in cooperation with the Mine 
Safety and Health Administration (MSHA) 
and the Bureau of Mines, has modified and 
tested the system at a variety of mines. 

Presently, the seismic location system 
is operational and deployed as one ele- 
ment of MSHA's Mine Emergency Operations 
(MEO) facility located near Aliquippa, 
Pa. This facility is maintained and 
operated by Westinghouse under MSHA con- 
tract. Following a mine disaster in 
which it is believed personnel are 
trapped underground, and it has been 
determined that the seismic location sys- 
tem may be necessary, the system is 
driven overland or transported by cargo 
aircraft to the mine disaster site. The 
system is then positioned over the 
suspected underground entrapment area. 



51 










FIGURE 1. - Seismic system for locating trapped miners. 



Figure 2 shows the van housing the seis- 
mic location equipment; in this case the 
van is mounted on a flat-bed vehicle. 
Hopefully, the deployment area is clear 
and readily accessible, but if not, it 
can be cleared by bulldozers and the sys- 
tem transported to the site by tracked 
vehicles or, if necessary, by helicopter. 
It is recommended that in order to pro- 
vide the best possibility of detecting 
and locating any trapped miners, the geo- 
phones be placed surrounding their most 



likely location. If the trapped miner is 
not in the area covered by the geophones, 
he or she may still be detected and lo- 
cated, but accuracy in the calculation of 
his or her location may suffer (6). How- 
ever, it is not necessary for the van to 
be placed in this immediate area owing to 
the large lengths of cable available to 
link geophones to the van; also, there 
exists the option of connecting the 
geophones to the van via a wireless 
frequency modulated (FM) telemetry link. 



52 




FIGURE 2. - Seismic van. 



In fact, it is recoaunended that the van 
or any other vehicle or personnel activ- 
ity be positioned far enough away from 
the geophones so that this activity would 
not interfere with the reception of any 
trapped miner seismic signals. 



The miner is instructed to do the follow- 
ing in the event he or she is trapped 
underground: 

1. When all possible escape is cut 
off, the miner is to barricade for 



After a site is chosen, the seismic 
array is deployed in a configuration that 
will cover the area to be monitored. An 
ideal array configuration is shown in 
figure 3. The array geometry is adjusted 
to the geometry of the mine and to sur- 
face conditions. The array consists of 
seven subarrays; each subarray is com- 
posed of either 7 or 24 geophones con- 
figured as shown in figure 4. Detailed 
discussion of these subarrays is given 
in a later section. While the array 
is being deployed, a survey of the sub- 
array locations is made using surveying 
equipment maintained with the seismic 
system. 



Array radius 



MSHA maintains a continuing effort to 
explain to the mining community the oper- 
ation of the seismic location system. 



FIGURE 3. - Ideal array configuration. 



53 




• • 



3. After hearing these three shots, 
the miner is to pound 10 times on any 
hard part of the mine, preferably the 
roof or a roof bolt, with any heavy ob- 
ject that can be found; a heavy timber is 
best. (Figure 5 shows a miner pounding 
while an operator in the seismic van ob- 
serves the geophone signals.) 

4. Following this, the miner is to 
rest for 15 min and then repeat the 
pounding. While resting, if the miner 
hears five shots from the surface he or 
she knows the signaling has been detected 
and help is on the way. 

5. If the miner hears no shots, he or 
she repeats signaling every 15 min. 



The above instructions are summarized 
in a hardhat sticker (fig. 6) that MSHA 
distributes. 



4.5nn 



During the expected signaling period, 
attempts are made to reduce surface 
activity while the seismic system is 
in use to optimize the chances of detect- 
ing the miner's signal. The system oper- 
ates continuously, but this quiet 
period should enhance the chances of 
detection during the expected signaling 
sequence. 



FIGURE 4. - Subarray configurations of (^) 24 
and {B) 7 geophones. 

protection from possible toxic gases and 
wait for a signal from the surface before 
beginning to signal the seismic system. 

2. As soon as the system is in a state 
of readiness, the surface crew detonates 
three explosive charges that can be eas- 
ily heard by the trapped miner. 



Once the signal is detected and the 
miner's location has been determined, 
directions are given to the rescue team 
members to guide them in their rescue 
efforts. If a rescue team is unable 
to reach the trapped miner, a drill- 
ing rig is positioned over the site of 
the miner's location and a rescue 
borehole is drilled for his or her 
evacuation. 



54 




FIGURE 5. - Surface personnel listening while a miner pounds. 



WHEN ESCAPE IS CUT OFF 



1. BARRICADE 



2. LISTEN 



1. BARRICADE 

2. LISTEN for 

3 shots, then .. 

3. SIGNAL by 

pounding hard 
10 times 



REST 15 minutes, 

then REPEAT signal until.. 

YOU HEAR 5 shots, which 
means you are located 
and help is on the way 

FIGURE 6. - MSHA hardhat sticker 




^m 



System Instrumentation 

The operation of the system may be best 
described by referring to the system dia- 
gram as shown in figure 7. The heart of 
the system is the electronic instrumenta- 
tion contained in the van, as seen in 
figure 8 in block diagram form. An in- 
terior view of the operating panel may be 
seen in figure 9. The geophone used is 
the Geospace GSC-llD model M-3,4 having a 
natural undamped frequency of 14 Hz. At 
each subarray, a preamplifier increases 
the signal level before transmitting it 
to the van via a cable or telemetry link. 
At the van, the signals are each passed 
separately through a tracking digi- 
tal notch filter. This filter removes 
narrow-band manmade interference such as 

'^Reference to specific equipment is for 
identification purposes only and does not 
imply endorsement by the Bureau of Mines. 



Supply trailer 



C 



Personnel 

transportation 

vehicle (s) 



C 



Iz 



iz 



Seisnnic 
instrumentation van 



7^ 



7\ 



FIGURE 7. - System block diagram. 



55 



^ 



Connmunications 
van 



O 



Power 

generating 

vehicle 



Oscillograph 



Bandpass 
filter 



Amp. 



Input 



7 channes 



Digital 
notch 
filter 



Time code 
gen.- trans. 



Tape 
search 



Tape deck 
I 



Tape deck 
2 



Analog 

to 

digital 

con- 

vertor 



Hard 
copy 
unit 



CPU 



Trigger 
scope 



CRT 
terminal 



Digital 
storage 



Program 
storage 



FIGURE 8. - Block diagram of van instrumentation. 



56 




FIGURE 9. " Interior view of van. 



powerline pickup or seismic disturbances 
caused by local machinery. This filter 
operates by latching onto the fundamental 
frequency of interference and tracking it 
if slight variations in frequency occur. 
This initial processing step eliminates 
interference that would, in some in- 
stances, limit system performance. 



After notch filtering the signals are 
amplified and then recorded on analog 
tape. The signals are band-pass filtered 
from 20 to 200 Hz and are displayed on an 
oscillograph record for recordkeeping 
purposes. By visually monitoring the 
oscillograph record, the operator can 
determine whether a signal has occurred. 



An example of the performance of the 
filter may be seen in figure 10. Fig- 
ure 10,4 shows a seismic record heavily 
contaminated with a 60-nz interference. 
Figure 105 shows the same record after 
passing through the notch filter and 
illustrates how a miner's signal is eas- 
ily seen after filtering, whereas prior 
to filtering it would have been impossi- 
ble to detect the signal. 



When the operator detects a signal, the 
analog tape containing the event is re- 
played into a PDF 11/34 computer via an 
analog-digital converter. The computer 
performs interactive signal processing on 
the data and displays the results on the 
computer's CRT terminal. A permanent 
record may be obtained using the hard 
copy unit. 



57 




Interaction even with the automatic de- 
tection system. 

Seismic Noise 



0.08 




FIGURE 10, - Seismic record showing signal 
and noise before {A) and after {B) digital notch 
filtering. 

When processing has been completed, the 
relative arrival times of the signals 
from each channel are determined. These 
data, together with information on the 
location of the subarray and the velocity 
of seismic waves obtained by the refrac- 
tion surveys, are submitted to the com- 
puter location program to determine the 
trapped miner's location. 

The present system relies on the oper- 
ator's ability to determine when a sig- 
nal has occurred. Manual detection of 
the signal can be unreliable due to the 
low signal-to-noise ratio (SNR) often en- 
countered and the inability of the oper- 
ator to maintain peak performance over 
extended periods of time. At present a 
contract effort is underway that will im- 
plement an automatic detection capability 
into the seismic system. The automatic 
system will provide equal performance to 
that of the human observer by allowing a 
computer to search the data for suspected 
signals. There will always be manual 



Seismic noise can at times be a major 
problem when detecting small-level seis- 
mic signals. Since the signal from a 
trapped miner can be on the order of a 
few microinches per second (yips), a nor- 
mal background noise can obscure the sig- 
nal. Thus information is needed on the 
types of noise sources, the expected am- 
plitude ranges, and the amplitude vari- 
ation with frequency and time. 

Three common noise sources are typ- 
ically encountered in the field: (a) 
natural seismic background noise, (b) 
manmade seismic noise, and (c) man- 
made electromagnetic interference (EMI) 
coupled into the field equipment. 
Narrow-band manmade noise may be read- 
ily eliminated by use of the digital 
notch filtering techniques previously 
discussed. 

Since natural seismic noise tends to 
vary widely as a function of time, geo- 
graphic location, and frequency, it is 
not possible to make precise predictions 
of the noise at the site of some future 
mine disaster; thus the noise must be 
treated in statistical terms. For some 
purposes, however, it is sufficient to 
know the noise characteristics within 
fairly broad limits. 

Study of the miner-induced frequency 
spectra indicates that most of the signal 
energy is in the frequency band between 
20 and 200 Hz. These studies have also 
shown that the amplitude of the envelope 
of the seismic noise is often Rayleigh 
distributed (3-4). 

Theoretical Seismic Waveform 
Modeling P rocedure 

An analysis was performed to under- 
stand the factors that affect the seismic 
signal amplitude, waveshape, and spec- 
tra. Based on this analysis, a waveform 
modeling procedure (WMP) was developed to 
model seismic signals generated from 



58 



impacts on the surface of mine workings. 
The output of the WMP is the predicted 
voltage waveform produced as sensed by a 
geophone. A block diagram of the WMP may 
be seen in figure 11. The computations 
for each of the boxes are convolved to 
give a final theoretical waveform that is 
then compared with actual field test mea- 
sured waveforms. 

A maj or factor that determines the 
seismic waveform is the time-dependent 
force that the miner's implement (timber, 
pick, etc.) applies to the mine roof or 
floor. It can be shown, based on the 
work of Sung (21) that if the wavelengths 
are long, compared with a length charac- 
terization of the surface area of the im- 
plement that is in contact with the sur- 
face of the mine opening, the force the 
implement exerts on the surface is pro- 
portional to the amount the surface is 
displacing. 



To include the effect of 
nel or cavity, the theory 
Greenfield (12) is used, 
spreading is given for the 
ation by a I/R dependence 
of anelastic attenuation 
Q-dampening) on the wave as 
is included by using 
operator (9). The effect 



the mine tun- 
described by 
The geometric 
present situ- 
The effect 
(often called 
it propagates 
the Futterman 
of geological 



Force time function 




I Loyering end free surface \^ 




Observed 



layering and the free surface of the 
earth is included by the method developed 
by Haskell (13), using a modification of 
the program described by Lablanc (17). 
The transfer function between the ground 
displacement and the voltage output of 
the seismic sensor was calculated based 
on the description of a seismometer given 
by Bollinger (J^) . The seismic system's 
20- to 200-Hz filter response was ob- 
tained by recording the impulse response 
of the filter. 

The waveform given by the WMP gives ex- 
tremely good fit to records observed at 
various field tests of the system. Both 
the waveshapes and the absolute ampli- 
tudes are well fit. The first example is 
from the Orient No. 6 mine; figure 12 
shows the actual seismogram as compared 
with a seismogram predicted by the WMP. 



ROOF BUOW 



Observed 




FLOOR BLOW 



Observed 




FIGURE 11. - Theoretical waveform modeling 
orncedure (WMP). 



FIGURE 12. - Comparison of actual and theo- 
retical waveforms. 



59 



Figure 13 shows the effect on the wave- 
form of soil thickness (d). For no soil 
(d = m) , the waveform is a single sim- 
ple pulse. For a thick soil layer 
(d = 20 m) , the waveform is a series of 
pulses of decreasing amplitude. These 
represent the successive bounces of the 
pulse owing to multiple internal reflec- 
tions in the soil layer. The time be- 
tween pulses is the two-way traveltime in 
the layer. As the layer thickness de- 
creases, the time between pulses de- 
creases; when d is reduced to 10 m the 
pulses overlap in time. 

The exact form of the signal is quite 
dependent on d. For this reason, the 
earth at any particular site is con- 
sidered as being comprised of a homoge- 
neous rock region and a soil layer near 
the surface. Recent work has indicated a 
need to compensate for this effect. Pre- 
liminary setup of the system now in- 
cludes a refraction survey that is 
usually conducted at every subarray lo- 
cation. The results of this survey de- 
termine the soil layer thickness, soil 
layer velocity, and rock velocity. These 
parameters are later used in the location 
algorithm. 



Soil thickness, m 



20.0 



10.0 



5.0 



2.5 








FIGURE 13. - Effect of soil layer thickness on 
vertical waveforms. 



Signal Amplitude Model for Various 
Sources 

In this section, signal sources are 
compared with the best-source type. This 
is done by relating the signal amplitude 
from other sources to that of the best- 
source type. From the data, an average 
difference in decibels (db) between each 
of the source types and the best-source 
type is determined. This difference is 
called the adopted value. 

In the majority of tests the best sig- 
nal source was a large timber applied to 
a roof bolt (denoted as source type SI), 
There are exceptions to this; for exam- 
ple, it was noted that at the Staufer 
Mine a large timber on the roof (there 
were no roof bolts) created weak signals 
owing to the height of the roof, which 
made it difficult to use the large timber 
effectively. However, in general, the 
large timber on the roof was either the 
best or within a few decibels of being 
the best-source type. Thus for the SI 
source, a value of db is adopted. 
Table 1 gives the adopted value for a 
variety of sources, 

Subarray Performance 

The seismic rescue system uses an array 
composed of seven subarrays rather than 
seven individual geophones to receive 
seismic signals, the reason is that a 
subarray will give a better SNR than a 
single geophone. This improvement is 
achieved principally in three ways. 
First, noise that is uncorrelated between 
the geophones will be reduced in ampli- 
tude by the cancellation that occurs when 
zero mean random numbers are averaged. 
Second, noise that is propagating at a 
slow horizontal velocity will be reduced 
on the output of the subarray because, if 
the subarray is thought of as an antenna, 
the noise will be outside of the anten- 
na's main beam. Finally, any adverse 
effects that would result if a single 
badly planted geophone was used will be 
alleviated by the averaging of all the 
subarray geophone outputs. 

As mentioned before, two subarray con- 
figurations have been developed aad used 



60 



TABLE 1. - Signal amplitude of various sources relative to signal amplitude of a 
large timber on a roof 



SI 



S2 



S3 



S4 



Source type. 



Application point. 

Orient y/6 Mine. db. . 

Peabody Mine db. . 

Peabody Mne db. . 

Concord Mine db. . 

Staufer Mine (no roof bolts).. db.. 
Value adopted for C^ db. . 

Source type 

Application point 

Orient #6 Mine db. . 

Peabody Mine db. . 

Peabody Mine db. . 

Concord Mine db. . 

Staufer Mine (no roof bolts).. db.. 
Value adopted for C^ .db. . 



Large timber. 

Roof bolt. 

NAp 
NAp 
NAp 
NAp 
NAp 




Small 

timber. 
Roof bolt. 
-7 
-3 
-1 
ND 

-3 



Sledge. 

Roof bolt. 
-8 
ND 
-3 
-3 
-1 
-3 



Large 

timber. 

Floor. 

-14 

-12 

ND 

-4 

+2 

-8 



S5 



S6 



S7 



S8 



Small timber. 
Floor. 

ND 
-16 
ND 
-4 
ND 
-10 



Hard hat. 

Roof bolt. 

-19 

ND 

ND 

ND 

-11 

-15 



Sledge. 

Floor. 

ND 

-14 

ND 

ND 

ND 

-15 



Rock pick. 
Roof bolt. 

ND 

ND 

ND 

-7 

ND 

-7 



NAp Not applicable. ND No data. 
^No roof bolts. 

^Approximate average difference between amplitude from source type and amplitude 
from large timber hitting roof bolt. 



extensively. The first is a seven- 
geophone subarray ( 25 ) with a 4.5-m di- 
ameter, having the geophones wired in 
parallel. The second is a larger 24- 
geophone subarray with a 24-m diameter. 
This large subarray uses two series- 
connected strings of 12 geophones with 
the two strings connected in parallel 
(8^). The subarrays are shown in fig- 
ure 4. The electronic configurations of 
both subarrays are such that the sensi- 
tivity of the subarrays is well below 
even low levels of natural seismic noise. 
Thus the ability to detect and identify 
signals from an underground miner is 
determined by the seismic noise level. 

The use of a subarray will normally re- 
sult in some loss of amplitude compared 
with using a single geophone in measuring 
a signal from a miner hitting below 
ground. This signal loss is due to the 
fact that the signal is not exactly the 
same on each subarray geophone. For a 
miner directly below the subarray, the 
the signal is in phase at all geophones 
and the signal loss will be minimal. 



However, for sources horizontally offset 
from the subarray there is a phase shift 
(or, equivalently , an arrival time dif- 
ference) between the geophones. 

The noise reduction for incoher- 
ent noise results in a gain of 13.8 db 
for the 24-geophone subarray and 8.5 db 
for the 7-geophone subarray. Seismic 
noise that is completely incoherent is 
not the normal situation but occurs 
during rain. In this situation the 
noise level is high and thus the subarray 
gain is especially important. Field 
test results have verified that this 
gain occurs during rain. In areas 
with brush or high grass ground cover, 
the noise generated by the wind may 
also be essentially incoherent between 
geophones. The larger spacing be- 
tween geophones of the 24-geophone sub- 
array compared with the 7-geophone 
subarray enhances the possibility that 
the noise will be incoherent. 

In many situations the seismic noise 
may be highly coherent between the 



61 



subarray geophones; however, the subarray 
can still give noise reduction because 
the noise is not in phase between the 
geophones (2). 

The source of coherent noise may be 
wind acting on trees outside of the sub- 
array, distant traffic, machinery, or 
airborne noise. Much seismic noise at 
frequencies of 20 to 200 Hz is of low 
horizontal phase velocity, since it trav- 
els at an acoustic velocity (330 m/sec) 
or at seismic surface wave velocities, 
which are usually below 1,000 msec. 

From theoretical considerations of SNR 
improvement by the 24- and 7-geophone 
subarrays, it is to be expected that the 
24-geophone subarray would offer a sig- 
nificant SNR gain over the seven-geophone 
subarray. In an extensive series of 
field tests this was often the case 
(8^). Typical gains were 5 db for the 
7-geophone subarray and 10 db for the 24- 
geophone subarray. There were, however, 
some mines where the SNR of the two sub- 
array types were comparable. The seven- 
geophone subarray, however, may offer 
practical advantages in terms of de- 
ployment, where a clear area cannot be 
found to deploy the larger 24-geophone 
subarray. 

Probability of Detection 

It is desirable to determine the proba- 
bility that a surface array will detect 
an underground source. In the config- 
uration normally used, seven subarrays 
are placed on the surface to monitor a 
portion of the subsurface. A method has 
been developed to calculate the prob- 
ability that m subarrays or more, with 
1 <_ m < 7, will detect a miner's signal. 
The detection of a signal by one subarray 
may be sufficient to identify the signal 
as coming from an underground miner. 
However, identification can be more cer- 
tain if several subarrays can detect the 
signal. To locate, at least three sub- 
array detections are required, and five 
or more are desirable for accuracy. 

In the following three examples, the 
substrata volume being monitored is a 



right-rectangular prism with top at h^ — 
200 ft and bottom at h2 — 1,200 ft as seen 
in figure 14. This depth range is con- 
sistent with the fact that the majority 
of mines lie in this range. 

Figure 15 is the first example of the 
results of the calculations. This view 
shows an array containing seven subarrays 
with a 500-ft radius. The subsurface 
being monitored is a square having sides 
of 2,000 ft. For the large timber on a 
roof bolt source with no subarray SNR im- 
provement, one looks at the 0-db position 
on the abscissa to get the probability of 




Subsuface volume 
being monitored 



FIGURE 14. - Substrate geometry for calcula- 
tion probability of detection; triangles indicate 
subarray locations. 



1 



o 

I— 
o 



o 

>- 



00 

< 
m 
o 
a: 



4 - 



2 - 




-16 



1 1 J'^'^Z^^^-^-'z^ 

/ / ^/ / i / 

/ / 7 '7 / 


y: 


- / / / / ^ f / 
/ / / / ^ '>/ 
/ / / / / 


, .500' 
• • • 

• • 


t - 

2,000' 


^ 1 / /I /I / 1 1 


1 1 



-12 



+ 4 



02 



8-4 

C, db — ► 

FIGURE 15. - Probability of detecting m or 
more subarrays for 500-ft array radius and 
2,000-ft monitored square. 



16 



62 



ra or more subarrays detecting the signal. 
For example, the probability of m = 5 or 
more detecting the signal is 0.62 (index 
base 1.000). 

From table 1, the signal for a small 
timber on a roof bolt (S2) has C = -3 db. 
Thus one looks at the -3 db abscissa val- 
ue for a S2 source. Note that for any 
source type, the use of the subarrays 
gives approximately a +5-db improvement 
in SNR compared with the single sensor 
values. After a single subarray has 
detected a signal, the stacking of suc- 
cessive blows will also improve the SNR. 
If 10 blows are stacked, a 10-db improve- 
ment is commonly obtained. Thus, for the 
case of locating the source using stacked 
traces from the subarrays, the C = +15 db 
value applies for the large timber on the 
roof bolt. 

Thus, for large-timber sources for the 
figure 4/4 configuration, it is very 
likely that at least one subarray will 
initially detect the signal; after stack- 
ing, signals should be seen on the five 
or more subarrays that are desirable for 
accurate location. 

Figures 16 and 17 show corresponding 
results for the monitoring of a square 
having sides of 4,000 ft for array radii 
of 500 ft and 1,000 ft. Since a large 
area is being monitored, the detection 




C,db— ► 

FIGURE 16. - Probability of detecting m or 
more subarrays for 500-ft array radius and 
4,000-ft monitored square. 




+16 



C, db 



FIGURE 17. - Probability of detecting m or 
more subarrays for 1 ,000-ft array radius and 
4j000-ft monitored square. 



probabilities are 
2,000-ft square. 



lower than for the 



For the monitoring of the 4,000-ft 
square with an array centered at the 
center of the square, the effect on the 
detection probabilities of the array 
radius was examined. This was done for 
the value C = db; that is, for the best 
source with no array gain or stacking. 
Results are shown in figure 18. To 
obtain a signal from at least one sub- 
array, the use of the larger radius 
arrays is somewhat better. The reason 
for this is that for the 500-f t-radius 
array points on the boundary of the 
square will be a minimum of 1,500 ft 
horizontally removed from the nearest 
subarray. Thus, to have the maximum 
probability of detection, it is suggested 
that before a signal is found it might be 
best to use a 1 ,000-f t-radius array when 
monitoring such a large area. If con- 
ditions allow, after detection on a 
single subarray, it would then be de- 
sirable to move some of the distant 
subarrays to the vicinity of the de- 
tecting subarray and signal the trapped 



63 



O 

I- 

o 

lU 

H 
Ui 

a 
u. 
O 

>- 



m 

< 
m 
O 

a. 




500 



2,000 



1,000 1,500 

ARRAY RADIUS, ft 

FIGURE 18. = Probability of detection with m 
or more subarrcys versus array radius 
db), and 4,000-ft monitored square. 



(C = 



miner to repeat his or her signal to 
allow improved location. 

Next, the situation will be examined 
where the trapped miner is believed to be 
below a particular point. One subarray 
would be set directly above that point. 
The probability of detecting the miner 
can be calculated by fixing the subsur- 
face region to be monitored as a very 
small area directly below the central 
subarray of a 1 ,000-f t-radius array. For 
a source 500 ft deep, even a weak source 
with C = -10 db will be detected with 
0.85 probability. Noting that a 24- 
geophone subarray gives a 5- to 10-db SNR 
improvement, it is expected that a sub- 
array would probably detect the signal 
even for sources down to 2,000 ft. This 



high probability of detecting a source 
directly below a subarray is consistent 
with the fact that in field tests signals 
from sources directly below a subarray 
were consistently detected. 

It is instructive to observe the vari- 
ation in the probability of detection as 
depth is varied. These results are shown 
in figure 19. The probability of detect- 
ing a miner's signal was determined when 
using an array of a 1,000-ft radius over 
square areas of 0.5 and 1.0 mile on a 
side, for varying depth. Probabilities 
were determined for weak and strong 
sources with and without processing. 
This processing takes the form of stack- 
ing. Also considered was whether detec- 
tion is probable on one or more subar- 
rays (m < 1) or five or more subarrays 
(m < 5). 

The detection probabilities discussed 
have all been based on the use of sub- 
arrays made of geophones that measure the 
vertical particle velocity of the ground. 
Geophones that measure the horizontal 
particle velocity are also manufactured 
and have been used in a limited number of 
experiments. The results of these exper- 
iments indicated that most often the ver- 
tical geophones outperform the horizontal 
geophones. There have been exceptions to 
this where the horizontal geophones have 
given better performance. To employ hor- 
izontal geophones, two extra channels 
(one for north-south and one for east- 
west polarization) at each subarray loca- 
tion must be employed. When using hori- 
zontal geophones each geophone must be 
carefully oriented. The signals from 
horizontal phones are often more diffi- 
cult to interpret. Therefore, the logis- 
tics of the operations suggest that for 
the surface seismic location system the 
present vertical geophone system should 
be maintained rather than a mixed verti- 
cal and horizontal system. 

Location Accuracy 

To guide the efforts of the rescue team 
or to determine where to site the rescue 
drill, it is necessary to determine the 
location of the trapped miner. For the 



64 



o 

LlI 

I- 

LU 
Q 

U_ 

o 

>- 

H 



CD 
< 
GD 
O 

CC 

CL. 



1.25 



1.00 



.75 



.50 



.25 - 



-Strong source with processing- 
Strong source and weak- 
source with processing 



"Weak source 



m > I 

Array radius= 1,000 ft 

Test area = 0.25 sq mi 



500 



1,000 
DEPTH, ft 



1,500 



2,000 



>- 

_j 
CD 
< 
CD 
O 

cr 

Q_ 



1.00 



.75 



.50 



.25 




500 



-Strong source and weak 
source with processing 



m 

Array radius =1,000 ft 

Test area = I sq mi 



-Weak source 



1,000 
DEPTH, ft 



,500 



2,000 



1.00 



.75 



H 
LLl 
Q 

U. 
O 

>- 
I- 



00 

< 

CD 
O 



.50 - 



.25 - 



c 


„^^ ^strong source with processing 


_ 


m > 5 




Array radius =1,000 ft 




Test area = 0.25 sq mi 


- 


Strong source and weak 




/source with processing 


- 


^Weak source 
1 f 1 1 



500 



1,000 
DEPTH, ft 



1,500 



2,000 



UJ 
Q 



CD 
< 
CD 
O 

or 

Q. 



WW 




D 


1 1 1 

Strong source with—^^^ ^ — "^ 
processing ^ 


.75 






/ m > 5 


.50 


- 




/ Array radius = 1,000 ft 
/ Test area = 1 sq mi 


.25 


- 




Strong source and weak 
source with processing "N^ 




^.--^ Weak source-^ 
1 1 1 N. 



500 



1,000 
DEPTH, ft 



1,500 



2.000 



FIGURE 19. - Probability of detection versus depth, array radius 1,000 ft. A, xx\ > 1, test area 
0.25 square mile; 5, m > 1, test area 1 square mile; C, m > 5, test area 0.25 square mile; /->, 
m > 5; test area 1,000 ft. 



65 



rescue team, an accuracy of 100 ft or 
less would appear desirable. For posi- 
tioning the drill an accuracy of a few 
feet would be desirable. However, as 
discussed below, accuracies of a few feet 
do not appear feasible. Thus, the posi- 
tioning of a rescue drill so as to inter- 
sect a mine entry near the estimated lo- 
cation of the trapped miner could best be 
done using a mine map, if available. 

The seismic system presently uses the 
"MINER" program (11) to calculate the 
location from arrival times measured on 
stacked seismograms. This program com- 
bines the individual subarray arrival 
times, either three or four at a time, to 
determine a location. The MINER program 
can use a known depth for the source or 
can fit for the source depth. Alternate 
methods of location based on the least 
squares principle are often used in seis- 
mic location work; this principle is the 
basis of work done by Ruths (19). 

Westinghouse (25) conqjiled estimates of 
location errors obtained for a limited 
number of locations at 12 mines. Table 2 
gives these results. This table indi- 
cates that horizontal location errors are 
usually below 100 ft. However, it should 
be understood that these results are gen- 
erally for the better SNR events and that 
the majority of the sources were located 
near the center of the array where loca- 
tion accuracy is best. 

TABLE 2. - Number of mines with average 
horizontal error in four ranges 

Number of mines within 
Error range, ft error range 



0-50 

50-100... 
100-200., 
Over 200. 



150 ft. In addition, extensive work by 
Ruths (19) showed errors of this order 
of magnitude for Island Creek's Hamil- 
ton No. 1 Mine. The mines at which 
the larger errors occur tend to have 
topographic relief and geologic condi- 
tions that vary with position. Ruths' 
work (19) indicates that the presence of 
very low velocity solid layers that are 
different between the subarrays is among 
the most serious sources of error. 

Three techniques have been used to de- 
crease the location error resulting from 
soil layer variations. Results to date 
with these techniques indicate that soil- 
layer-related errors can be reduced to 
100 ft or less. Ruths ( 19 ) studied the 
first of these techniques both by com- 
puter simulations and by study of data 
from an earlier Island Creek Mine field 
test (24). 

In technique 1, called the reference- 
correction method, it is necessary to get 
a source to within several hundred feet 
of the suspected position of the trapped 
miner. This might be impractical in a 
disaster situation. As an alternative 
method of employing technique 1, a re- 
ceiver in a drill hole near the level of 
the mine might be used to measure travel- 
times from shots near each subarray. 
The reference-correction method appears 
to greatly improve the probability that 
location errors will be below 100 ft even 
in mines with highly variable near- 
surface conditions. In technique 2, a 
short refraction measurement is made at 
each subarray and used to make an arrival 
time correction. In technique 3 an 
arrival time is measured at each subarray 
from a blast at a known position outside 
the seismic array. Recent work at the 
Hamilton No. 1 Mine (March 1980) gives an 
indication that the errors from soil lay- 
er variations can be greatly decreased by 
use of technique 2 or 3. 



Two of the mines discussed by Westing- 
house had average errors of approximately 



66 



THE P]M LOCATION SYSTEM 



Parallel to the seismic location pro- 
gram, the Bureau has maintained an EM 
location research effort. The EM tech- 
nique offers the potential of providing a 
superior locatioa method along with the 
capacity for voice communications. Dur- 
ing the past 12 yrs, the Bureau has di- 
rected this program to the point of 
developing hardware prototypes and con- 
ducting performance evaluation, implemen- 
tation assessment, and reliability stud- 
ies. In addition, existing research 
projects include alternate EM techniques 
involving computerized signal processing. 
These alternate methods would make sig- 
nificant improvements in performance, 
which would allow implementation of EM 
location devices in very deep mines where 
the present EM or seismic methods are not 
feasible. 



Concept 



The premise on which the EM system 
would be implemented is that the trapped 
miner would deploy a small transmitter 
that would be powered by a cap lamp bat- 
tery. This transmitter would be con- 
nected to a length of wire, forming a 
magnetic antenna. The resulting magnetic 
field would then be detected on the sur- 
face and the trapped miner's underground 
location ascertained. The surface per- 
sonnel could then establish voice com- 
munications to the miner (downlink); how- 
ever, limited power underground would 
preclude the use of voice uplink communi- 
cations. This concept may be seen in 
figure 20. 




FIGURE 20. - Through-the-earth transmission system. 



67 



Early in the EM research program the- 
oretical and experimental studies have 
shown that the best chance for success 
was in a system comprised of a narrow- 
band transmitter and receiver. Initial 
units of this type were developed by 
Collins Radio (4), with an improved ver- 
sion built by General Instrument Corp. 
(20), as shown in figure 21 connected to 
a modified cap lamp battery. The major- 
ity of these units were constructed in 
the belt-worn configuration, as shown in 
figure 22; a few were constructed di- 
rectly into a cap lamp battery. The 
antenna is included in this package and 
consists of 300 ft of No. 18 copper wire. 
Four frequencies have been chosen and 
are 630 Hz, 1,050 Hz, 1,950 Hz, 3,030 Hz. 
Included in several of the units are 
baseband receivers capable of receiving 
voice communications from the surface. 

The surface equipment consists of 
narrow-band, personnel-carried receivers 
in conjunction with hand-held antennas. 
The narrow-band receivers and antennas 
exist in helicopter-borne versions also. 
Aerial searches are performed to deter- 
mine the general locale of the signal and 
are used when the mine presents large 
surface areas to be searched. Surface 
crews are then used to provide a more 
accurate location. A high-power audio 





FIGURE 21. - General Instrument transmitter 
mounted on a modified cop lamp battery. 






FIGURE 22. - General Instrument transmitter. A, packaged belt-worn configuration; B, package 
cover removed exposing antenna spool. 



68 



amplifier and large loop of wire are 
included in the surface equipment. The 
amplifier provides the capability of 
voice downlink communications. 

Although the trapped miner cannot re- 
spond with voice communications, the 
transmitter is equipped with an on-off 
key. This key may be used for responding 
to the downlink voice communications in a 
coded fashion. In this configuration and 
with the cap lamp turned off, the trans- 
mitter will continue to operate for a 
period of 2 to 4 days, depending upon the 
state of discharge the battery was in 
when the miner became trapped. Figure 23 
shows the life expectancy of the cap lamp 
battery when operating the transmitter. 
The range of values were obtained using 
an old battery with an 8-hr discharge to 
an upper bound of an undischarged new 
battery. A 2-ohm resistor was used to 
simulate the antenna load. 

As mentioned previously, the prototype 
units built by General Instrument were 
mainly in the belt-worn configuration, 
even though the method of eventual imple- 
mentation into the mining community has 
not yet been fully assessed. Other meth- 
ods of deployment are being considered; a 



few examples are fixed-position deploy- 
ment at strategic locations, mounting 
units on vehicles, having only foremen 
carry them, and building them directly 
into the cap lamp battery. 

EM Experimentation 

Early in the EM research program, the- 
oretical efforts were undertaken to in- 
vestigate the surface fields created by a 
subsurface buried magnetic antenna (14). 
These formulations assumed a homogeneous 
earth of conductivity. The conductiv- 
ity serves to attenuate the signal as 
it propagates through the earth. Addi- 
tional theoretical work has been done to 
include the effects of a stratified 
earth. However, at these wavelengths, 
the homogeneous half -space model is usu- 
ally sufficient. 

The research also included an extensive 
field testing program. The objective of 
the 94 field tests was twofold. First, 
the tests were to define a signal trans- 
mission and analysis program to obtain a 
reliable data base for characterizing the 
signal transmission properties of over- 
burdens in the U.S. coal fields and, 
second, to use this data base to predict 



T 



T 



T 



T 



T 



New battery, fully charged 




40 50 60 

TIME, hr 

FIGURE 23o - Cap lamp voltage variation with time while operating trapped miner transmitter. 



69 



the likelihood of successful performance 
of the EM trapped miner location system. 

The mines sampled for these tests were 
selected from a population of all coal 
mines on the basis of both the overburden 
depth and number of miners employed in 
the mine. The sample reflected concern 
both for the physical dependence of sig- 
nal penetration on depth and the number 
of miners exposed to potential disasters 
within each depth interval. Figure 24 
shows the cumulative distribution of 
mines as related to maximum depth and 
demonstrates that approximately 90 pet of 
all U.S. coal mines are less than 
1,000 ft deep. 

The field testing was conducted by 
Westinghouse (10) and Bureau personnel. 
Data analysis was performed by Arthur D. 
Little, Inc. (16). These data are pres- 
ently still being analyzed by the Bureau 
attempting to more accurately assess 
through-the-earth EM propagation by uti- 
lizing more complicated mathematical mod- 
els to describe the data. The Bureau 
also regularly conducts EM field tests to 
further supplement this data. 

The two most important factors that in- 
dicate how well a signal will propagate 
through the earth are the overburden 
bulk conductivity and the mine depth. 
Unfortunately, the geological structure 
of the overburden above coal mines dif- 
fers from mine to mine, which causes the 




500 



1,000 1,500 

MINE DEPTH, ft 



2,000 2,500 



FIGURE 24. - Cumulative distribution of coal 
mine depths throughout the United States. 



electrical conductivity to vary also. 
Therefore, for a given mine depth, one 
would expect the signal transmission 
characteristics to vary from one mine to 
the next. In order to predict the signal 
strength at any mine, one must rely on a 
statistical assessment of the data and 
then to use this statistical data to 
determine signal strengths at any mine 
based on depth alone. 

The root mean square (RMS) values 
of the vertical magnetic field, H, of all 
of the data taken were normalized 
to a transmitter magnetic moment of M = 1 



amp 



Following this normalization. 



statistical studies were performed to re- 
late the surface field strength and mine 
depth at each frequency tested. 

Each normalized data point can be 
denoted as S|j, where the subscript i 
represents the specific frequency and 
thus the subscript j represents the spe- 
cific depth of test for each mine. Thus, 
each surface measurement, Sj =, taken can 
be considered as a single observation of 
the signal strength at a predetermined 
frequency and overburden depth level at a 
particular mine. The selection of the 
mines tested was done on a statistically 
based random sample to assure that S 
could be described by a common 
mal probability law. 



J 
nor- 



Several linear regression models were 
hypothesized and tried. The model found 
to best fit the behavior of the data is 
one in which the mean value of the nor- 
malized signal strength, Sj ,, is linearly 
related to the logarithm of overburden 
depth. This is shown in equation 1, 



S, ■ = a, + 3i log (depth) + e. 



(1) 



Here S| j is the normalized vertical mag- 
netic field signal strength (expressed in 
db re 1 pamp/m-RMS for the ith frequency 
and depth j for a transmit moment of 
M = 1 amp-m^). 

The parameters a^ and 3| are parameters 
to be estimated from the data, where 
depth is known in meters. The parameter 



ij 



represents a random variable that is 



70 



normally distributed, with expected value 
zero and variance which is the same for 
all values of j . 

The derived regression lines for each 
of the four frequencies are plotted in 
figure 25. It is visually apparent that 
the log-linear relationship is an appro- 
priate one and the R^ statistic, a mea- 
sure of goodness of fit, supports this 
observation. 

Two types of intervals have been esti- 
mated from the data. One is known as a 
confidence interval, which is defined as 
a range of values computed from the sam- 
ple that can be expected to include the 
true (but unknown) mean value with a 
known probability. Figure 26 displays 
95-pct confidence intervals with dashed 



lines. To illustrate this concept using 
figure 26^4 , it follows from the field ex- 
periment that the probability is 95 pet 
that the interval from -6 to -12 db in- 
cludes the true mean normalized signal 
strength for a transmitter of magnetic 
moment M = 1 amp-m^ at 630 Hz and an 
overburden depth of 190 ft. 

While the confidence interval repre- 
sents a probability statement about a 
mean value over many trials it is also 
of interest to quantify the expected out- 
come of a single trial. For example, 
what signal strength could be expected if 
a test were conducted at a predeter- 
mined frequency and overburden depth? 
This situation is depicted by predic- 
tion intervals also plotted in figure 26. 
To illustrate this concept, again using 



LOG (depth, ft) 
200 300 400 500 700 1,000 1, 500 



1 \ \ 1 \ ^ 

Surface vertical magnetic field, H^, versus log (depth) for 
Q transmit moment M=l amp-m^ 



Amount of variobilify not 
explained by the Ime 




Regression line 

20 log S= 97 62 - 6 1 .97 log (depth) 

R^= 83 pot 



Amount of variability 
explained by the line 



KEY 
o Observed dato values 



Mean of 
observed values 



J L 



J L 



.2 13 14 15 16 17 18 1.9 2.0 21 22 23 24 25 26 27 
LOG (depth, m) 



30 
20 

10 

-10 
-20 
-30 
-40 
-50 
-60 
-70 

-80 

I. 



200 



LOG (depth, ft) 
300 400 500 700 



1.000 



gression line 
20 log 3 = 108.01-6711 log (depth) 
R^=85 pet 



KEY 
o Observed data values 



1.500- 



1 \ \ 1 \ T 

Surface vertical magnetic field, Hj, versus log (depth) for _ 
a transmit moment M = l amp-m^ 




2 1.3 1.4 1.5 16 17 18 19 2 2 1 22 23 2,4 2 5 2 6 27 
LOG (depth, m) 



200 



300 



LOG (depth, ft) 
400 500 700 1.000 1.500 



1 I I \ T 

Surface vertical magnetic field, H^, versus log (depth) for 
a transmit moment M=l amp-m^ 




30 

20 
10 


-10 
-20 
-30 
-40 
-50 
-60 - 
-70 



LOG (depth, ft) 
300 400 500 700 1.000 



I 1 I \ r 

Surface vertical magnetic field, H;, versus log (depth) for 
transmit moment M= I omp-m^ 




Regression line 

20 log 3 = 12494-76.71 log(depth) 

R^80 pet 



KEY 
o Observed data values 



J L 



J I I I L 



13 14 15 16 IT 1.8 1.9 2.0 2 I 22 2 3 2.4 2 5 2 6 2 7 
LOG (depth, m) 



FIGURE 25. - Uplink normalized overburden signal response data and linear regression log (depth) 
modeL A, at 630 Hz; B, at 1,050 Hz; C, at 1,950 Hz; D, at 3,030 Hz. 



71 





30 




20 


F 




\ 




a. 


lU 


E 




o 




a. 





aj 










-10 


J3 




-o 








^ 


-^0 


I 




X 


-^0 


H 




ID 






40 


fr 




1- 




(/). 


■50 



y-eo - 



-80 
-90 



S.^^^^^ ^ 1 ! i 1 I'll ' 1 . 1 1 1 1 1 1 
_ \. Normalized to transmit moment of M=l amp-m 


*v \^^ \. /-SS pet prediction interval 
\^ "^^^^V^X^ ^\ r Regression line 






^Free-space curve 

"0\ 








95-pct / \^_^ 

prediction interval \ 


/ 


^ ^K 


95-pct confidence Internal 






1 III,, 




. 1 ..!,,., 



10 ^C 



E 
o 
S. 



~^-20 
X 

X-30 
CD 

S-^0 



'"-50 - 



y-60 





1 1 1 1 ' ' ' 1 ' 1 ' 1 1 
Nornnalized to transmit moment of M=l amp-m^ 


1 I I 1— 


\ 


^\ ^95-pcf prediction interval 


- 


--> 


"^^^'^^^^ ^Regression line 


- 


\ 


v^^ ""^^^^^^ J^\^"^^ ^Free-space curve 


- 


- 




- 


- 


^\X>^ 


- 


- 


95-pct ^ ^\ ^^^^STV. ^"^^ 


- 


- 


prediction interval ^^^ >Vn.- 


;^ 






w "^ ^ 




/^N. ^ 








^x\^ 




95-pct confidence interval ^\^ 










- 


1 1 1 1 1 < 1 1 1 1 . 1 1 





100 150 200 250 300 500 700 

OVERBURDEN DEPTH, ft 



100 150 200 250 300 500 700 1,000 

OVERBURDEN DEPTH, ft 



1.500 



30 

20 

E 

a '0 

E 

o 

a. 



X 

X-30 
(D 
^-40 

a: 

t- 
to-so 

Q 

y-60 



Z)-80 
en 





C 1 ' ' ' 


' ' 1 


1 1 I 1 


I'll 


"■" 


\ 


V. Normalized to transmit momen 


f of M=l amp 


-m^ 


- 


s. 












s 


%x .^^ 


-pet prediction Interval 






\ 




.«„ 


ression line 




- 


- 


\. ^-N^v ^ 


?^~X 


^Free-space curve 


- 


- 


— ^ 


^t^ 


^C 




- 




95-pct -^ 
prediction interval 






^^ 










\«. 


^\^ 
■^^x 


*%> 




95-pct confidence 


interval 






s 










V X 




- 


1 III 


1 , 1 


1 1 1 1 


1 1 1 1 





20 

E 
E 



~m"20 
X 

X-30 

h- 

u-40 
a: 

1^-50 
Q 

y-60 



D 


1 1 1 1 ' ' ' 1 ' 1 




>\ 


Normalized to transmit moment of M=l amp-m^ 


- 


>i 


^\ ^95-pct prediction interval 


- 


X. 


"~v\^^"\ ^v ^Regression line 

\^ ov^ "/\^v ^Free space curve 


- 


- 


95 pet ^ \. ^>^ ^>^ 

prediction interval \. ^^N^ ^W 

^s. tS^ ^^ 


- 


- 




"s 


- 


95-pct confidence internal ^v. ^^^\^ >. 


- 




\ 



200 250 300 500 700 

OVERBURDEN DEPTH, ft 



1.500 



150 200 250 300 500 700 

OVERBURDEN DEPTH, ft 



1.000 



1.500 



FIGURE 26. - Uplink regression results, normalized vertical signal strength, hertz versus depth. 
A, for 630 Hz; B, for 1,050 Hz; C, for 1,950 Hz; D, for 3„030 Hz. 



72 



figure 26, the probability is 95 pet that 
another test performed at 630 Hz at a 
depth of 500 ft would yield a signal 
strength between -49 and -22 db. Also 
plotted in figure 26, for comparison, is 
a curve of the free space vertical field 
strength that would be measured on the 
surface in the absence of the lossy over- 
burden media. 

Figure 27 summarizes the normalized 
average overburden response as a function 
of depth and frequency by plotting the 
four regression lines and the free space 
curve on the graph. This figure shows 
that the frequency dependence of signal 
strength is relatively insignificant for 
depths less than 500 ft, and that the 
change across the band is only about 
10 db even at the maximum depth of 
1,500 ft. 

These summary normalized overbur- 
den response plots, together with the 



Normalized fo transmit moment ot M=l amp-m'^ 

\ 
\ 
\ 
\ 

Free- space curve 




\ "S;; 1,050- 



1,950 



1^1 3,030 



150 200 250 300 400 500 700 1,000 

OVERBURDEN DEPTH, ft 



1,500 2.000 



FIGURE 27, - Normalized overburden response 
curves. Uplink regression results, average sur- 
face vertical signal strength, hertz versus over- 
burden depth by frequency. 



confidence and prediction levels of this 
section, can be used to generate esti- 
mates of signal strength produced on the 
surface above coal mines as a function of 
overburden depth and operating frequency 
for transmitters having any prescribed 
magnetic moment versus frequency charac- 
teristics in the 630- to 3,030-Hz band. 

EM Noise 

Magnetic field noise raeasureraents were 
obtained during the course of the mea- 
surement program. This set of data was 
obtained by a Bureau team performing 
noise analysis of 27 of the 94 mines 
tested. The Bureau's data were gathered 
on tape and later analyzed in the labor- 
atory. For purposes of signal detecta- 
bility, the RMS value of the vertical 
magnetic field is of interest. The sta- 
tistical distribution of this noise, 
using the Bureau data base, at each fre- 
quency for a receiver bandwidth of 30 Hz 
is shown in figure 28. 

Surface SNR 

In previous sections, the behavior of 
signal data and noise data obtained in 
this study have been characterized by 
statistical relationships. To develop an 
understanding of detection probability it 
is necessary to characterize the proba- 
bility distribution of the surface RMS 
SNR at each frequency. 

The indepencence of signal and noise 
distributions, in addition to the prop- 
erty of normality exhibited by each 
distribution, permit straightforward com- 
bination of the two distributions to 
generate SNR probability estimates. By 
the central limit theorem, the sum (or 
difference) of two normally and inde- 
pendently distributed variables is also 
normally distributed. 



73 





A 


1 


1 1 ! 1 1 ' 


1 1 


1 1 1 


1 
/ 












/ 




- 






OCX 


/ 




- 


- 




Meon - 4.3 


/o 






- 










~ 






^ 






~ 


~ 




1 1 /I 1 


1 1 1 1 1 1 1 


1 1 


1 1 1 


I 



O £ 



40 


- 


B 




1 1 


1 1 


1 1 1 


1 1 


1 1 


1 1 1 


1 


30 
20 


- 














7 




- 


10 


- 












f" 


> 




- 









Mean= 


-1.8 




^ 


4 






- 












■^ 


10 






1 


5^ 

1 ^1 


1 1 


1 
1 

1 

t 1 1 


1 1 


1 1 


1 1 1 


1 



2 5 10 20 30 40 50 60 70 80 90 95 98 99 99-5 99.9 
CUMULATIVE NORMAL PROBABILITY, pel 



1.0 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.9 

CUMULATIVE NORMAL PROBABILITY, pet 




5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.9 

CUMULATIVE NORMAL PROBABILITY, pet 



liJ lo 



- 


D 


1 [ 


1 1 


1 1 1 1 1 


I 


1 1 


1 1 


1 


- 














/o 


- 


- 












9°/^ 




- 


- 




Mean = 


-17.1 




r 


Y 




- 










^L^O 


- 




1 1 


P\ 1 


A6\ 

1 1 1 1 1 


1 


1 1 


1 1 1 


1 



1.0 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99 5 99.9 

CUMULATIVE NORMAL PROBABILITY, pet 



FIGURE 28- = Statistical distribution of RMS surface noise at (.-1) 630 Hz, {B) 1„050 Hz, ((') 1,950 Hz; 
(/;) 3,030 Hz. 



The SNR distributions are conveniently 
plotted using normal probability paper. 
Such normal probability plots derived are 
given in figure 29 for five different 
overburden depths at each of four 
frequencies . 

These four graphs provide a straight- 
forward method to estimate the proba- 
bility of having various SNR's in actual 
practice. The vertical axis represents 
the area under the normal curve from 
minus infinity to some SNR, R^^ , and pro- 
vides the probability of achieving a SNR 
less than or equal to R^ . 

It is instructive to observe the be- 
havior of probability estimates associ- 
ated with exceeding a given SNR as a 
function of overburden depth and fre- 
quency. Figure 30 gives the the proba- 
bility of the RI4S signal being at least 



greater than RMS noise. Note that the 
best performance occurs in the upper part 
of the frequency band even though more 
loss occurs through the earth at the 
higher frequencies and the magnetic mo- 
ment is smaller at the higher frequen- 
cies. This can be explained owing to the 
rapid decrease in surface noise levels as 
frequency increases. 

Signal De t ecti on Criteria 

The success of rescue effort when using 
a trapped miner transmitter rests on the 
ability of surface personnel to confi- 
dently detect the signal from the under- 
ground transmitter. The pulsed signals 
from the underground transmitters are 
detected by searchers carrying rescue 
receivers equipped with a hand-held loop 
antenna and headsets. The mode of detec- 
tion is aural, based on the headset 



74 





-20 -15 



5 10 
R„=SNR, db 




5 10 
Ro= SNR, db 



98 


1 
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1 1 1 


1 1 1 


1 L 


/ 


97 


- 






y/ 


- 


96 


— 






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95 


- 






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7 


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/ ~ 


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/ 




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J 


q: 








y/ 


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7 




. 


> 












< 10 


— / 








7 


_i 










/ 


r) 










/ 


5 








J 


/ 


3 5 












2 
1 


1 


L/ 1 1 


, , / 


1 1 


- 



-20 -15 -10 -5 



5 10 
R„=SNR, db 



15 



20 25 30 



FIGURE 29. = Cumulative probability distribution of SNR's expected above U.S. underground 
coal mines at {A) 630 Hz; (/i) 1.050 Hz; (C) 1,950 Hz, [D) 3,030 Hz, 



75 



— "°^v**^^i^5:-rK ' ' 


1 


\^>x 


KEY 


XX.. t 


— 1,950 Hz 
— D 3,030 Hz ~ 
— A 1050 Hz 


\ V'v - 


630 Hz - 




s 


" 


^^XX" 


1 1 1 1 


1 



250 500 750 1,000 

OVERBURDEN DEPTH, ft 



1,250 



1,500 



FIGURE 30. - Probability that mean RMS sig- 
nal is greater than or equal to RMS noise +9 db 
for General Instrument transmitter. 

signals perceived by the ear. It is then 
necessary to establish a relationship 
between the nature of the signal, SNR, 
and the probability of aural signal 
detection. 



The pulse length is also an important 
aspect of signal detectability. Psycho- 
acoustic data taken by a number of in- 
vestigators determined the "recognition 
differential" required versus pulse 
length for a 50-pct probability of detec- 
tion. The recognition differential is 
the amount in decibels by which the sig- 
nal level needs to exceed the measured 
noise spectrum level (noise level in 1 Hz 
within critical band of interest) to 
provide a 50-pct probability of detec- 
tion. The General Instrument (GI) trans- 
mitters have a fixed pulse duration of 
100 msec which prescribes a recognition 
differential of 23 db to achieve a 50-pct 
probability of detection. To determine 
the significance of the 23-db recognition 
differential in terms of required SNR, a 
bandwidth must be chosen. The bandwidth 
used in the receiver is 30 Hz, one-half 
of the critical bandwidth of the ear at 
the listening frequency. 



The aspects of the signal that influ- 
ence detection are (a) frequency, (b) 
signal length, and (c) signal repetition. 
The primary aspect of the noise for 
detection considerations, besides the 
level of the noise, is the noise band- 
width. How each of these parameters 
affects the signal detection capability 
must be understood, then their results 
can be combined to generate a probability 
of detection curve as a function of 
SNR. 

The present receiver mixes the received 
signal with an internal oscillator to a 
higher frequency for purposes of narrow- 
band filtering, then mixes the filtered 
signal again to present a listening sig- 
nal of 978 Hz to the operator. The abil- 
ity to detect a tone masked by broad-band 
noise as a function of frequency has been 
studied by Urick (22). 

When the ear listens for a tone, it 
acts as a narrow-band filter centered at 
the signal frequency. The bandwidth of 
this apparent narrow-band filter is known 
as the critical bandwidth. The band- 
width is approximately 60 Hz at the 
978 Hz listening frequency of the rescue 
receivers. 



Studies ( 18 ) have shown that systems 
with bandwidths approximately one-half 
the critical bandwidth will behave in the 
same manner detectionwise as those having 
a system bandwidth equal to the critical 
bandwidth. Therefore, for purposes of 
the trapped miner system, a SNR of 23 
-10 log 30 = 8 db is needed to yield a 
50-pct probability of detection. 

A final factor affecting detection is 
the signal repetition rate. Garner ( 10) 
provides data on the effect of the repe- 
tition of a pulsed tone on signal detect- 
ability. According to this work the 1-Hz 
repetition rate of the trapped miner 
transmitter should require 2 db less SNR. 
The 50-pct probability of detection SNR 
criterion of (8 - 2) db or 6 db, will be 
used. 

This work quantifies the necessary SNR 
to establish a 50-pct detection probabil- 
ity. It is also necessary to extend this 
work to determine detection probabilities 
at any other SNR. The results of this 
extension are shown in figure 31. This 
plot can be used with the earlier estab- 
lished expected SNR for the underground 
transmitter to establish signal detection 
probabilities. 



76 



100 



80 



< O 60 

m p 
^a 40 



20 



6.0 db at 
50 pet probability 



1.5 2.5 3.5 4.5 5.5 6.5 7.5 

SIGNAL-TO-NOISE RATIO, db 



8.5 



10.5 



FIGURE 31. - Aural probability of detection ver- 
sus RMS SNR for trapped miner pulsed continuous- 
wave signals in background Gaussian noise. 

Probability of Detection Estimates 

In an actual mine emergency situation 
many factors will influence the actual 
ability to rescue the miner. Time of 
arrival of the rescue team, life expect- 
ancy of the miner, search times, and 
operation time of the underground trans- 
mitter are only a few of the factors that 
have a bearing on the success of the res- 
cue effort. This report has not dis- 
cussed these points but rather has in- 
vestigated the detection probability for 
an existing signal as being measured by a 
rescue team in an area which in general 
is directly over the trapped miner. Even 
within this measurement there are factors 
such as geology, noise, and depth that 
influence the probability of success. 
However, though these factors may not 
enable the success of this measurement to 
be stated in a deterministic manner, 
the chances, as outlined in this paper, 
can be quantified in a probabilistic 
framework. 

The probability of detection curve in 
figure 31 actually represents a condi- 
tional probability; that is, the likeli- 
hood that detection will occur given the 
presence of a fixed RMS SNR. As a conse- 
quence, the chance of detecting a signal 
transmitted through the earth can be cal- 
culated according to 

P {D and R,^} = P {R^} x P {dIr^}, (2) 

where {D and R|^} represents the probabil- 
ity of achieving a SNR of size R^ and 



also detecting the signal embedded in the 
noise. P {R|^} is the probability of the 
occurrence of a SNR of the size R|^ and 
P {d|r,^} is the conditional probability 
of detecting a signal given a SNR of size 
R|^. 

The results of these calculations pre- 
sent, as shown in figure 32, the expected 
property of detection estimates for GI 
transmitter signals as measured over all 
the U.S. coal fields. 

SUMMARY 

A system based upon seismic techniques 
as envisioned by the NAE in 1970 has 
proven to be an effective means for de- 
tecting and locating miners trapped un- 
derground following a mine disaster. 

Expected signals from miners pounding 
on the roof of a mine are of sufficient 
strength to enable detection over a large 
area of the mine. Estimations of the 
location of the trapped miner are of suf- 
ficient accuracy to aid the rescue team 
or the positioning of a rescue drill. 

The seismic system, as discussed in 
this report, is presently operational and 
in a state of readiness in the event of a 
mine disaster. It should prove to be an 
invaluable aid to future postdisaster 
rescue efforts. The attractiveness of 
this technique is that it requires no 



O 8 



- — ^ 


^^==^-^^ 1 1 1 1 




\. ^^^'^t-".. 


' 


\ \3. 


— 


\ \V KEY 




\ \~V 1.950 Hz 

\ \ "^.^ ° -03,030 Hz 


" 




- 


\ \ \X O 630 Hz - 

\ \ ^N 


- 


\ \\-- 




\v^. 




^^\V "■>- 


. 


\^*^ \^-. 




^^^^ ^^ 








"^^-"-^J"^^ X ^ 




-o^^ "y 




1 1 1 1 1 T 



250 



500 750 1,000 
OVERBURDEN DEPTH, ft 



1,250 



1,500 



FIGURE 32. - Predicted probability of signal 
detection versus overburden depth by frequency 
for the General Instrument transmitter. 



77 



active devices to be carried by under- 
ground miners. The components necessary 
for utilizing this method are readily 
available in any mine. A limitation of 
the seismic location system is that it 
provides no communication capability. A 
detailed technical discussion of the 
seismic system is contained in a report 
by Durkin and Greenfield (7^). 

This paper has also outlined the EM 
trapped miner communications and location 
research program conducted at the Bu- 
reau's Pittsburgh (Pa.) Research Center. 
It has also discussed the extensive field 
testing program to evaluate the trans- 
mitter performance. Analysis of this 
data ( 15) has enabled one to place into a 
probabilistic framework the ability to 
confidently detect the signal from the 
underground transmitter. Results indi- 
cate that the probability of detecting 



this signal is 45 pet at a depth of 
1,000 ft, a depth which exceeds 90 pet of 
the coal mines in the United States, and 
a 90 pet probability at a depth of 
500 ft, a depth which exceeds 50 pet of 
the mines. This information is vital for 
the future formulation and promulgation 
of new regulations written for the use of 
the EM system. 

Studies are currently underway to im- 
prove the detection capability by provid- 
ing signal processing capability in the 
receiver. Future work will look at a 
systems approach when using this tech- 
nique. This study will investigate each 
element involved in a successful rescue 
effort, such as research strategies, life 
expectancies, etc. Coupled with the re- 
sults discussed in this paper, a thorough 
understanding of the effective implemen- 
tation of the EM system will be obtained. 



REFERENCES 



1. Anema, C. Waveform Generator- 
Package and Receiver (Mancarried and Hel- 
icopter Receiver Portion) (Contract 
H0242010, Collins Commercial Telecomaiuni- 
cation Div.). BuMines OFR 74-78, Novem- 
ber 1976, 54 pp. 

2. Bollinger, G. Blast Vibration 
Analysis. Southern Illinois Press, Car- 
bondale, 111., 1971, pp. 37-45. 

3. Capon, J., R. J. Greenfield, R. J. 
Kolker, and R. T. Lacoss. Short-Period 
Signal Processing Results for the Larger 
Aperature Seismic Array. Geophysics, 
V. 33, 1968, pp. 452-472. 



6. Crosson, S., and D. C. Peters. 
Estimates of Miner Location Accuracy: 
Error Analysis in Seismic Location Pro- 
cedures for Trapped Miners. Pt. 3 in 
Survey of Electromagnetic and Seismic 
Noise Related to Mine Rescue Commun- 
ications. Volume II. Seismic Detec- 
tion and Location of Isolated Miners. 
(Contract H0122026, A. D. Little Inc.), 
BuMines OFR 38(2)-74, January 1974, 
pp. 3.1-3.36. 

7. Durkin, J., and R. J. Greenfield. 
Evaluation of the Seismic System for Lo- 
cating Trapped Miners. BuMines RI 8567, 
1981, 55 pp. 



4. Capon, J., R. J. Greenfield, and 
R. T. Lacoss. Long-Period Signal Pro- 
cessing Results for the Large Aperature 
Seismic Array. Geophysics, v. 34, 1969, 
pp. 305-329. 

5. Committee on Mine Rescue and Sur- 
vival Techniques, National Academy of 
Engineering. Mine Rescue and Survival. 
Final Report (Contract S0190606) Bu- 
Mines OFR 4-70, March 1970, 81 pp.; NTIS 
PB 191 691. 



8. 



Study of Possible Modifica- 



tions to the Trapped Miner Seismic Loca- 
tion System. Unpublished Bureau of Mines 
report (Interim Rept. 4268), May 15, 
1978, 95 pp.; available for consultation 
at Bureau of Mines Pittsburgh Research 
Center, Pittsburgh, Pa. 

9. Gutterman, W. I. Dispersive Body 
Waves. J. Geophys. Res., v. 67, 1962, 
pp. 5279-5291. 



78 



10. Garner, W. R, Auditory Thresholds 
of Short Tones at a Function of Repeti- 
tion Rates. J. Acoustical Soc. Am., 
V. 19, No. 4, July 1947, pp. 600-608. 

11. George, D. C, and R. F. Linfield. 
Seismic Subsystem Location Calculation: 
Software Concepts and Interpretation. 
Sect, in Trapped Miner Location and Com- 
munication System Development Program. 
Volume 1. Development and Testing of an 
Electromagnetic Location System. (Con- 
tract H0220073, Westinghouse Electric 
Corp.), BuMines OFR 41(1 )-74, May 1973, 
pp. G1-G23. 

12. Greenfield, R. J. Seismic Radi- 
ation From a Point Source on the Surface 
of a Cylindrical Cavity. Geophysics, 
V. 43, 1978, pp. 1071-1082. 

13. Haskell, N. A. Critical Reflec- 
tion of P and SV Waves. J. Geophys. 
Res., v. 67, 1962, pp. 4751-4767. 

14. Hill, D. A., and J. R. Wait. Ana- 
lytical Investigations of Electromagnetic 
Location Schemes Relevant to Mine Rescue 
(Contract H0122061, Inst, of Telecom- 
munications Sci.). BuMines OFR 25-75, 
Dec. 2, 1974, 147 pp. 

15. Kehrman, R. F., A. J. Farstad, D. 
Kalvels. Reliability and Effectiveness 
Analysis of the USBM Electromagnetic 
Location System for Coal Mines, Final 
Report (Contract J0166060, Westing- 
house Electric Corp.). BuMines OFR 
47-82, Dec. 1, 1978, 153 pp.; NTIS 
PB 82-201385. 

16. Lagace, R. L, J. M. Dobbie, T. E. 
Doerfler, W. S. Hawes, and R. H. Spencer. 
Detection of Trapped Miner Electromag- 
netic Signals Above Coal Mines (Contract 
J0188087, Arthur D. Little, Inc.). Bu- 
Mines OFR 99-82, July 1980, 281 pp.; NTIS 
PB 82-244732. 

17. Lablanc, G. Truncated Crustal 
Transfer Function and Fine Crust- 
al Structures Determination. Bull. 
Seismic Soc. of America, v. 57, 1967, 
pp. 0719-0734. 



18. National Defense Research Center, 
Division 6. Principles and Applications 
of Underwater Sound. Summary Tech. 
Rept., V. 7, Washington, D.C., 1946, rev. 
1968. 

19. Ruths, M. A. The Reference- 
Correction Method for Improving Accuracy 
in the Seismic Location of Trapped Coal 
Miners. M. S. Thesis, Pennsylvania State 
Univ. , College of Earth and Mineral 
Sciences, University Park, Pa., November 
1977, 141 pp. 

20. Simmons, C. H. Development and 
Prototype Production of a Trapped 
Miner Signaling Transmitter/Transceiver 
(Contract J0395017, Gen. Instrument 
Corp., Government Systems Div.). Bu- 
Mines OFR 95-82, June 1981, 82 pp.; NTIS 
PB 82-244260. 

21. Sung, T. Y. Vibrations in Serai- 
Infinite Solids Due to Periodic Surface 
Loading. Paper in Symposium on Dynamic 
Testing of Soils. American Society for 
Testing and Materials, Philadelphia, Pa., 
1953, pp. 35-63. 

22. Urick, R. J. Principles of Under- 
water Sound for Engineers. McGraw Hill 
Book Co., Inc., New York, 1967. 

23. Westinghouse Electric Corp. Coal 
Mine Rescue and Survival. Volume 2. 
Communications Location Subsystem (Con- 
tract H0101262). BuMines OFR 9(2)-72, 
September 1971, 268 pp.; NTIS PB 208 267. 



24. 



Field Tests — Seismic Loca- 



tion System, Mine Emergency Operation 
Group [MESA (MSHA) Contract J0277500]. 
March-October 1976, 403 pp. Available 
for consultation at the Bureau of Mines 
Pittsburgh Research Center, Pittsburgh, 
Pa. 



25. 



Mine Emergency Operations 



Program Seismic Location Field Test Pro- 
gram [MESA (MSHA) Contract J0177500]. 
April-September 1977; available for con- 
sultation at the Bureau of Mines Pitts- 
burgh Research Center, Pittsburgh, Pa. 



79 



BUREAU OF MINES BOREHOLE PROBES PROGRAM 
By James R. Means, Jr. 



ABSTRACT 



The Bureau of Mines has developed 
probes for deployment through boreholes 
drilled into mines for the purpose of re- 
mote information retrieval. Various 
probes provide closed-circuit-TV mon- 
itoring, two-way voice communications. 



temperature measurements , and batch gas 
sampling. These probes can provide ac- 
curate information about the mine envi- 
ronment when access into the mine is 
impossible. 



INTRODUCTION 



Communication with miners and informa- 
tion about environmental parameters are 
essential to the safe operation of any 
mine. However, following a disaster 
(when information is most needed), the 
normal paths of communication into the 
mine are usually disrupted, and obtaining 
data about the mine and communicating 
with the miners are impossible. Conse- 
quently, the Bureau of Mines has devel- 
oped several types of borehole probes, 
each capable of establishing a telecom- 
munications link into the underground en- 
vironment via boreholes drilled into the 
mine. 



sites for borehole drilling, and drilling 
should commence at the earliest possible 
time since the drilling of a single bore- 
hole may take several days. 

Applications to data have been in three 
separate but related categories: 

1, Location of trapped miners, 

2, Collection of environmental data 
following a mine disaster, 

3, Diagnostic work in mine subsidence 
efforts. 



These cylindrical probes are lowered 
into the mine via a combination strength 
and communication cable. Although the 
probes are by nature limited to obtaining 
information in the immediate vicinity of 
the borehole, they can reach locations 
inaccessible by conventional means. To 
maximize the utility of the probes, care 
must be exercised in the selection of 



Capabilities of existing probes include 
closed-circuit TV, two-way voice communi- 
cation, remote gas sampling, and remote 
temperature readout. Currently the 
closed-circuit-TV capabilities are being 
upgraded, and additional probes are being 
considered for high-temperature mine fire 
applications. 



TV PROBE 



The oldest of the Bureau's probes is 
the TV probe, which was developed for use 
in trapped-miner detection but has also 
found use in mine subsidence efforts and 
even in a shaft inspection. Figure 1 is 
a drawing of the probe. 

The TV probe features a low-light-level 
TV camera, which was developed for the 

'Electrical engineer, Pittsburgh Re- 
search Center, Bureau of Mines, Pitts- 
burgh, Pa. 



National Aeronautical and Space Admin- 
istration (NASA) by Westinghouse. This 
camera utilizes a silicon-intensif ied- 
target (SIT) vidicon, which enables it to 
function at faceplate illuminations as 
low as 10"^ foot-candles. Focusing and 
iris control are accomplished via a cus- 
tom remote-controlled lens, and the view 
orientation of the camera is converted 
from downward to horizontal via a 
45° mirror placed just below the camera 
lens. 



80 



^ 



n 



L-r-l 



Power supply -control module 



^- Explosion - proof housing 



M 



^^ 



Electromechanical 
deptoyment cable 



Rotator 



Camera 



Mirror 

-Pop-off shutter 
Light source 



Bottom switch 



FIGURE 1. - Mark III television probe. 

The camera, the remote lens, and an 
NiCd battery pack to power the probe are 
encased in an explosion-proof housing to 
mitigate any potential explosive hazard 
associated with these components. This 
configuration has prevented wires from 
being routed to the bottom section of the 
probe. Consequently a self-contained 
light section was designed. 



The light section consists of a miner's 
cap lamp mounted for horizontal illumina- 
tion and a NiCd battery pack. This 
arrangement provides illumination suit- 
able for viewing at distances of up to 
70 m (220 ft). Focusing of the cap lamp 
is done manually before the probe is 
deployed. 

The probe can rotate through a full 
360° via a rotator section in the top of 
the probe. This section contains a 
motor-gear unit which rotates the bottom 
of the probe with respect to the top sec- 
tion. Double-armored cable, which is 
used in TV probe applications, prevents 
the top section from rotating. Thus the 
bottom section rotates as the top remains 
stationary. 

To minimize dirt in the optics of the 
system, a spring-loaded pop-off shuttle 
is placed over the cutaway sections of 
the main probe housing. When the probe 
hits the bottom of the mine, a bottom 
switch activates a screw-drive mechanism 
which releases the spring-loaded shutter. 
This removes any dirt from the optical 
path that has been accumulated during the 
descent of the probe. 

This probe is large, measuring 291 cm 
(9 ft 6-5/8 in) tall by 8.9 cm (3-1/2 in) 
in diameter, and weighing approximately 
135 pounds. It is deployed via a double- 
armored cable which has a break strength 
of several thousand pounds. The cable 
contains 13 conductors and an RG59 co- 
axial cable (coax). The probe uses five 
wires and the coax. Connection of the 
probe to the cable is done by a machined 
stainless steel marine-type connector 
which provides mechanical strength as 
well as electrical connection. 

The TV probe is deployed from a custom- 
built truck-winch system which was con- 
figured to fit on a C-120B airplane 
(fig. 2). This allows for rapid deploy- 
ment in emergency situations. The winch 
was fitted with approximately 2,000 ft 
(615 m) of cable, and a 6.5-kVA generator 
was mounted on the truck to provide power 
for the probe and associated equipment. 
The truck-probe system has been success- 
fully deployed and was delivered to the 



81 




FIGURE 2. - Custom-built truck-winch system. 



Mine Safety and Health Administration's 
(MSHA's) Mine Emergency Operations (MEO) 
group for use in postdisaster operations, 

A. new version of the above probe is 
currently under construction for the 
Bureau of Mines2 that will extend the 
capabilities of the TV probe. This will 
include the addition of an electronic 
compass with remote readout, a remote 
zoom lens, and multiplexing of all 



control signals onto the video coax. All 
original probe functions will be re- 
tained, and the outside diameter will be 
increased to 10 cm (4 in) . 

Additionally, a new section will be 
provided that will permit downward view- 
ing (fig. 3). This unit will require a 
26-cm (10-in) hole and should be useful 
in shaft emergencies. The new probe 
should be completed in fiscal year 1983. 



COMMUNICATIONS PROBE 



Technical and regulatory constraints on 
the explosion-proof housing of the TV 

^Design Engineering Laboratories, Tor- 
rence, Calif. 90505, Bureau of Mines 
Contract HO308041, Closed Circuit TV 
Borehole Probe. 



probe made inclusion of voice communica- 
tions impossible. Consequently, a sepa- 
rate probe was designed and constructed 
to meet this need. This probe is much 
smaller than the TV probe and can be 
deployed by a hand-operated winch, as il- 
lustrated in figure 4. 



82 



Cop lamp (2 required) 

- Sheet meiol housing 




Access doors os required 



FIGURE 3. - Downhole-viewing module. 

Circuitry for this probe is simple and 
intrinsically safe. The transmit cir- 
cuitry is contained in the uphole control 
unit, and the speakers are in the probe. 
Receiver circuitry consists of a micro- 
phone with a solid state amplifier which 
transmits a signal to a receiver in the 
uphole control unit. Transistor radio 
batteries (9-V) are located in both the 
probe and the control box for powering 
circuitry. 

A flashing array of LED's is located at 
the bottom of the communications probe 
as a visual indication of the probe's 
presence. This circuitry is pow- 
ered by its own 9-V transistor radio 
battery. 



(T^ 



Headphones 



Receiver- transmitter 




14 cm 

Scale 



Speaker - microphone 
array 



Flashing light 



FIGURE 4. - Communications probe. 

The communications probe operates 
at voice frequencies and is of the 
push-to-talk type of operation. No 
plans exist to improve this probe, which 
is capable of operation at sufficient 
depth for use in any U.S. mine. This 
probe was also delivered to MSHA's MEO 
group. 



BATCH GAS-SAMPLING PROBE 



Traditionally, gas-sampling remote ar- 
eas of a mine following a disaster has 
been done through a plastic tube lowered 
into the mine via a borehole. This can 
be done accurately, but owing to stretch- 
ing of the tube, the user never is sure 
of the depth of the end of the tube. It 
is also good practice to have a redundant 
reading to verify data. Consequently, a 
batch gas-sampling probe was developed 
for MSHA by the Bureau of Mines with a 
bottom indicator to assure samples are 
taken within the mine. 

Figure 5 shows the batch gas-sampling 
probe and its control unit. Three 



separate vacuum bottles are contained in 
the probe. Each of these can be punc- 
tured individually with a hypodermic 
needle driven by a motor-worm gear unit 
which will automatically retract. This 
permits a gas sample to enter the bottle, 
which can later be analyzed. The sensor 
on the bottom will give an indication 
that the probe has reached the mine floor 
by turning on a light-emitting diode on 
the control box. These two functions 
allow batch gas samples to be taken accu- 
rately in the mine. 

Remote temperature monitoring was also 
included in this probe. This was done 



83 




FIGURE 5c - Batch-sampling probe and its control. 



with a thermocouple and an electronic ice 
point reference junction. This allows 
the reference voltage to be transmitted 
along copper conductors where it is read 
out in a digital readout in the control 
unit. 



The probe has been tested in both field 
and laboratory tests and has been ac- 
cepted by MSHA. It is currently deployed 
at HSHA's MEO facility. No plans exist 
for upgrading this probe at the present 
time. 



CONCLaSIONS 



The Bureau of Mines has developed bore- 
hole probes capable of gaining visual in- 
formation, taking gas samples, indicating 
temperature, and establishing voice com- 
munications through boreholes into a 
mine. Data gained from these probes can 
be useful in locating trapped miners or 
dealing with mine problems if the bore- 
holes are located properly and drilled 
in a timely manner. These probes are 



currently under the jurisdiction of MSHA 
at the MEO facility. 

The Bureau is currently upgrading capa- 
bilities of the TV probe to include a re- 
mote compass, a remote zoom lens, multi- 
plexing of all controls, and downward 
viewing. These functions will be con- 
tained in a new probe scheduled for com- 
pletion in fiscal year 1983. 



84 



MINE PERSONNEL LOCATOR AND IN-MINE ACTIVITY CONTROLLER 

By James R. McVeyl 



ABSTRACT 



The Bureau of Mines, through contract 
J0205059 with Nelson and Johnson Engi- 
neering, Inc., Boulder, Colo., has devel- 
oped the design for a personnel locator 
and in-mine activity controller. The new 
system, when fabricated, will provide 
mine management immediate access to the 
location of underground personnel and 
enable in-mine monitoring and control. 
The inability to quickly determine the 
location of underground personnel and 
control critical underground activities 
has always been a problem and generally 
hampers rescue operations in case of 
disaster. 

Although Public Law 91-173 states that 
each mine operator shall maintain a 
check-in, check-out system for iden- 
tifying persons underground, current 



identification systems provide no means 
of knowing where a miner is underground. 
Miners often leave their normal work sta- 
tions to provide other services. The 
personnel locator virtually eliminates 
this change of work station problem with 
its automatic monitoring capabilities. 
The system consists of a host (above- 
ground) computer, strategically located 
underground remote terminals, and cap- 
lamp transponders that automatically 
interrogate the miners any time they pass 
a remote terminal. Their location change 
is immediately transmitted to the surface 
to update the host computer information. 
The system will monitor personnel and 
equipment movement and has analog and 
digital input-output capabilities for 
measurement and control. 



INTRODUCTION 



The inability to quickly determine the 
location of personnel and control criti- 
cal underground activities generally ham- 
pers rescue operations during and after a 
mine disaster. Public Law 91-173 states 
that each mine operator shall maintain a 
check-in, check-out system for identify- 
ing persons who are underground. Nearly 
all mines today use a large board with 
numbered hooks and brass tags. When min- 
ers go underground, they remove their 
brass tags from the board and take them 
along; in some cases, magnetic nameplates 
are used with two-sided colors which the 
miners place on a personnel location 
board to indicate that they are under- 
ground. These indicators are returned to 
an out-of-mine position when the miner 
exits the mine. A very serious shortcom- 
ing exists in this procedure, in that 
miners often leave their normal work 

— , ^ 

'Supervisory electronics technician/ 

Spokane Mining Research Center, Bureau of 

Mines, Spokane, Wash. 



stations and there is no convenient 
recording method available to notify 
surface personnel of the change. Post- 
disaster information on the location of 
trapped miners has usually indicated that 
miners were found at locations other than 
their normal work stations and had not 
taken expected escape routes. 

The mine personnel locator and in-mine 
activity controller (MPLAC) has been 
designed to help eliminate this problem. 
The new system will automatically log the 
miners into the mine and keep track of 
their direction of travel and location 
underground. The system, which has been 
designed but not built, consists of a 
host computer, strategically located re- 
mote terminals, and cap-lamp transponders 
(transceivers). Each miner is automat- 
ically interrogated as he or she passes 
or enters the radio frequency (RF) field, 
which usually extends up to 200 ft from 
the remote terminal. The miner is iden- 
tified by an assigned code transmitted 



85 



from the cap-lamp battery transpoader to 
the remote terminal. The remote terminal 
retransmits this signal to the surface, 
updating the miner's location and direc- 
tion of travel in the next polling from 
the host computer. 

The system is also designed as a to- 
tal mine-monitoring system. The remote 
terminal not only monitors personnel and 
equipment locations, but is also equipped 
with analog and digital inputs and digi- 
tal outputs for measurement and con- 
trol. These input-output functions allow 



measurement of various parameters and 
activities such as ventilation, toxic 
gases, smoke or fire detection, and a 
host of others. The digital outputs 
allow control of alarms and equipment. 
An alphanumeric display and keyboard 
allow sending and receiving of messages 
between terminals and the host computer. 
Paging is also provided. A. visual page 
is displayed at the terminal and by a 
page indicator light on the miner's cap- 
lamp battery if the miner is within the 
terminal's transmitting range. 



UNITS OF MEASURE ABBREVIATIONS USED IN THIS REPORT 



ft 


feet 


sec 


seconds 


KHz 


kilohertz 


tpd 


tons per day 


MHZ 


megahertz 


V 


volt 


msec 


milliseconds 







SYSTEM DESCRIPTION 



Though equipment can purchased for mon- 
itoring many mine parameters, none can 
quickly determine the location of under- 
ground personnel. The system described 
in this paper, a combination of available 
mine-monitoring and computer components, 
provides a continuous update of personnel 
location. New features are the cap-lamp 
transponder and a remote interrogation 
terminal. 

The mine personnel locator (fig. 1) 
consists of the main host computer and 
data printer, interconnecting communica- 
tions data link (coaxial or fiber op- 
tics), remote terminals, and cap-lamp 
transponders. The host computer is a 
Columbia Products Commander Series 900.^ 
The Commander 900 was chosen for its 



industrial adaptability, memory, and 
input-output expansion capabilities; many 
other computers will function equally 
as well. The data printer is an Oki- 
data u80 and provides a hard-copy 
output of requested information. The re- 
mote terminal, yet to be built, is a 
microprocessor-based unit that provides 
automatic transponder interrogation, mine 
measurements, communications, and con- 
trol. The transponder, also yet to be 
built, is a small radio transmitter- 
receiver that is mini-dip-switch pro- 
grammed to the miner's identification 
code. The transponder is located in the 
hood that covers the top of the cap-lamp 
battery. Communication between the host 
and remote terminals is by coaxial or 
fiber optics cable, user's choice. 



HOST TERMINAL 



The Columbia Data Products Series 900 
microcomputer is responsible for the gen- 
eral control of all operations. Once 
programmed, it will continuously poll 

^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



all underground measurements and cause 
the remote terminal to produce control 
functions if programmed to do so. The 
computer is Z80 microprocessor-based and 
has 32K bytes of random memory (RAM), ex- 
pandable to 64k bytes. A dual mini- 
floppy-disk drive is built into the main 
frame, providing an additional 320K 



86 




Underground 
data terminal 



i=iiii=)iii=iiii=iiii=(iii=)TfT=iiri=r 

FIGURE lo - Conceptual view of mine personnel locator and mine activity controller. 



bytes. Multiple input-output options 
are available (fig. 2). The floppy 
controller can handle two additional 
external disk drives for further memory 
expansion. A cathode-ray tube provides 
visual display of all data requested 
for viewing by the operator. "Basic" 
computer language was chosen for the 



operating system for ease of programming 
and use by the industry. The computer 
easily provides control of all functions 
required for personnel location and 
underground measurement, plus many man- 
agement functions such as accounting and 
maintenance scheduling. 



REMOTE TERMINAL 



The remote underground terminal has 
been designed using an Intersil 87C48 
microprocessor and performs the following 
tasks: 

1. Reads and obeys keyboard data 
input. 



2. Displays messages from the host 
computer and other underground terminals 
via the host. 

3. Provides the host computer with up- 
dated transponder data. 



87 



Z80A 



Host system 



2K PROM 



32K-64K RAM 



Disk 
160K/320K/IM 



Optional disl< 
160K/320K/IM 



e[' 



Standard 
interfaces 



4 RS 232 ports 
1 with TTY 



~4 8-bit parallel 

I/O ports 
- (32 lines) 



rPrinter 
Paper tape 
Mag tape 
Other 



Standard counter 
timer circuit 4 in- 
dependent channels 



Optional arithmetic 
processing unit 



Available 

custom 

interfaces 



Optional 
" DMA 



Optional 
IEEE 



Winchester disl( 

.High-speed device 

D/A and A/D converter 

Voltmeters 

Mag tape 

Scanners 

Thermal sensors 

Instrumentation 

Other 



Optional 
user specific 
I/O 



FIGURE 2. - Commander block diagram. 

4. Continually interrogates trans- 
ponders in the area, 

5. Measures or interrogates all mea- 
surement channels (sensors). 

6. Outputs command signals (sets off 
alarms, etc.). 

7. Communicates with the host computer 
and other terminals via the host. 

The remote terminal (fig. 3) is power- 
ful enough to provide multiple functions, 
thereby relieving the host computer of 
all underground data retrieval duties. 
The remote terminal utilizes two 40- 
character lines for displaying messages 
readable from 20 ft. Through side- 
mounted connectors, it can measure up 
to eight 0- to 10-V differential analog 



inputs, 12 optically isolated contact 
closures, and one optically isolated 0- 
to 100-KHz frequency channel. Four dig- 
ital output connector channels (contact 
closures) are provided to set off alarms 
and control functions. The remote ter- 
minal sends out an interrogation pulse 
(RF signal) to check for miners in the 
area every 5 sec. This information is 
stored and polled by the host computer 
for updating miner location. A full 
alphanumeric keyboard and special func- 
tion switches provide data entry and 
retrieval. Communication between the 
host computer and terminal or terminal- 
to-terminal communications can be via 
coaxial or fiber optics cable. Figure 4 
depicts a miner using the remote termi- 
nal as a communication device. 

TRANSPONDERS 

The miner location transponder (fig. 5) 
is a small radio frequency transmitter 
and receiver (transceiver) located in the 
hood of the miner's cap-lamp battery. 
The transponder is totally automatic. 
The miner's recognition code is set into 
a mini dip switch mounted in the hood of 
the cap lamp and is a part of a timing 
circuit. Every 5 sec, a pulse is trans- 
mitted from the remote terminal and is 
received by all transponders in the area. 
When each transponder (fig, 6) receives a 
pulse, it starts a countdown to the count 
set by its own dip switch. When this 
count is reached, the transponder trans- 
mits a code back to the remote terminal, 
identifying itself within a time window 
determined by the dip switch. Each tim- 
ing window (fig, 7) is 20 msec in length. 
Therefore, 5 sec provides access to 256 
transponders (windows). By entering work 
shift codes into the computer, one can 
expand the number of total personnel to 
be monitored. The cap-lamp power cord 
serves as the antenna, and power is sup- 
plied by the cap-lamp battery, A small 
light-emitting diode, located on the lid 
of the cap-lamp battery, is turned "on" 
any time there is a page to be answered, 
RF transmission frequency has been set at 
49,6 MHz, 



Sand cast 

aluminum 

case 



Power 



Data bus 



Pressure-fitted 
bacl( plate 



Analog 
inputs 




Transponder 
transmit-receive 
antenna 



2-line, 80 character 
alphanumeric display 

1 -piece smooth surface 
control panel 

Digital inputs and outputs 
(hidden from view) 

128-character sealed 
ASCII keyboard 



FIGURE 4o - Remote terminal deployment. 



Note: All case penetrations are sealed; 
unit is pressurized, and charged 
with inert gas 

FIGURE 3. - Conceptual drawing of remote terminal. 

COMMUNICATION LINK 



The personnel locater can use either 
coaxial or fiber optics cable for a 
transmission media. Coaxial cable is 
much cheaper but it limits band width. 
Fiber optics transmission gives almost 
limitless band width for the system. Be- 
cause fiber optics cable is becoming more 
cost effective, it will probably be used 
as the communication link, with the first 
installation. 

SYSTEMS UTILIZATION 

The MPLAC has been designed to, hope- 
fully, combine the best of two worlds — 
mine personnel location and underground 
measurement. The new system will in- 
crease safety and emergency capabilities. 




89 




FIGURE 5. - Transponder-cap lamp and battery. 



Through proper and multiple use of stra- 
tegically located underground remote ter- 
minals, one can monitor the locations of 
all underground personnel and mobile 
equipment. These same terminals can also 
provide cost savings to the user through 
their automatic measurement capabilities. 
The miner can use the MPLAC to monitor 
conveyor belts, power, power factor, ven- 
tilation, methane gas, etc. The monitor- 
ing of power and power factor can reduce 
required feeder and transformer capaci- 
ties, thereby reducing overall costs. 
Fire hazards caused by overload or 



unbalanced loads (creating heat) are also 
reduced. 

The monitoring of belts can reduce 
maintenance problems and belt downtimes 
and thus increase production. Many mines 
have indicated that monitoring belts, 
alone, has offset the cost of the system 
the first year of operation. Monitoring 
power factor, phase loading, and holding 
down peak power demands during mine 
startup can bring about reduced power 
costs and more lucrative power contracts. 



90 



v«^ 







n 














RF 
demodu- 
lator 


Decoder 




-i-640 


Delay 
counter 




_j 






1(1, 


Enable 


19.5 
msec 
















RF amp 


Encoder 




TX 

timing 

logic 


Dip switch 

channel 

coding 




i_ 













FIGURE 6, - Transponder block diagram. 



5-sec 
timer 



Li 



5 sec 



^ ^ 



< h 



U 



Channel 
window 



Channel 1 
window 



Channel 2 
window 



■CZh 



-^20.0 
msec 



■cn 



-*i20.oh 

msec 



czu 



-*i20.0 
msec 



i b 



i ^ 



HZZh 



Channel 254. 
window 

Channel 255- 
window 



■^ h 



■i ^ 



k 



120.0 
msec 



■*120.0 
msec 



FIGURE 7. - Transponder timing— 256 channels. 



CONCLUSIONS AND RECOMMENDATIONS 



91 



Present mine-monitoring systems do not 
monitor miner's location. This concep- 
tual design provides this capacity. It 
is believed that the Mine Personnel Lo- 
cator and In-Mine Activity Controller, 
designed under Bureau of Mines contract 
J0205059 by Nelson and Johnson Engineer- 
ing of Boulder, Colo. , is a necessary and 
viable tool for the mining industry. The 
new system is state-of-the-art in design 
and versatility. It combines measure- 
ment functions and much-needed personnel 



location into one 
operate system. 



convenient, easy-to- 



The conceptual design is complete and 
ready for the hardware construction 
phase. The mining industry has reviewed 
the system during the design phase (ta- 
ble 1) and has provided much needed in- 
put. Acceptance has been good, and a 
tentative proposal for a 50-50 cost-share 
hardware phase has been received from one 
mine. 



TABLE 1. - Mining industry reaction to mine personnel locator and activity controller 



Mine 



Geneva. . . . 

Sufco 

Galena. . . . 

Highland. . 
Empire, . . . 
FMC 

Tenneco. . . 
Texas Gulf 



Type 



Coal. . . 
. . ,do, , 
Silver, 

, , ,do, , 
Uranium 
Coal, , , 
Trona, , 
• , ,do, . 
, , ,do. . 



Estimated 
size, tpd 



1,450 

9,000 

750 

1,000 
3,250 
3,000 

(2) 
4,800 
7,000 



Mine 
monitoring 



Neutral 

Very important 
Useful.. 

do 



Very important 

do 

Important 

Very important 



Digital 
communications 



Of limited value 

• ••••QO* •••••••• 



.do, 



Useful 

Not needed 

Neutral 

Not needed 

• ••••QO»«« •••••• 



Personnel locating 



^ These above respondents indicated that they would probably buy 
system by itself, if it were offered, and integrate it into their 
systems. 

^Tons per day size is proprietary; 6- by 6-mile area. 



Neutral. 
Useful. 

Useful but diffi- 
cult to implement. 
Do, 
Important, 
Very important, ^ 
Would not buy it. 
Useful. 

Ve r y_ iinpo rtant . 

a personnel locator 
existing monitoring 



T>U S GOVERNMENT PRINTING OFFICE; 



1982 - 605 - 015/98 



INT.-BU.OF MINES,PGH.,P A. 26491 



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